options: Search upwards for tags file #2670

Also remove some references to emacs tags from the docs.

References https://github.com/neovim/neovim/issues/1664

runtime/doc/eval.txt

@@ -6723,7 +6723,6 @@ digraphs		Compiled with support for digraphs.
 dnd			Compiled with support for the "~ register |quote_~|.
 dos16			16 bits DOS version of Vim.
 dos32			32 bits DOS (DJGPP) version of Vim.
-emacs_tags		Compiled with support for Emacs tags.
 eval			Compiled with expression evaluation support.  Always
 			true, of course!
 ex_extra		Compiled with extra Ex commands |+ex_extra|.

runtime/doc/options.txt

@@ -6501,8 +6501,7 @@ A jump table for the options with a short description can be found at |Q_op|.
 	tags file are relative to the directory where the tags file is.
 
 						*'tags'* *'tag'* *E433*
-'tags' 'tag'		string	(default "./tags,tags", when compiled with
-				|+emacs_tags|: "./tags,./TAGS,tags,TAGS")
+'tags' 'tag'		string	(default "./tags;,tags")
 			global or local to buffer |global-local|
 	Filenames for the tag command, separated by spaces or commas.  To
 	include a space or comma in a file name, precede it with a backslash
@@ -6519,10 +6518,6 @@ A jump table for the options with a short description can be found at |Q_op|.
 	|+path_extra| feature}
 	The |tagfiles()| function can be used to get a list of the file names
 	actually used.
-	If Vim was compiled with the |+emacs_tags| feature, Emacs-style tag
-	files are also supported.  They are automatically recognized.  The
-	default value becomes "./tags,./TAGS,tags,TAGS", unless case
-	differences are ignored (MS-Windows).  |emacs-tags|
 	The use of |:set+=| and |:set-=| is preferred when adding or removing
 	file names from the list.  This avoids problems when a future version
 	uses another default.

runtime/doc/tagsrch.txt

@@ -410,15 +410,6 @@ for.  In Vim this is not done, the previous search pattern is still remembered,
 unless the 't' flag is present in 'cpoptions'.  The search pattern is always
 put in the search history, so you can modify it if searching fails.
 
-					*emacs-tags* *emacs_tags* *E430*
-Emacs style tag files are only supported if Vim was compiled with the
-|+emacs_tags| feature enabled.  Sorry, there is no explanation about Emacs tag
-files here, it is only supported for backwards compatibility :-).
-
-Lines in Emacs tags files can be very long.  Vim only deals with lines of up
-to about 510 bytes.  To see whether lines are ignored set 'verbose' to 5 or
-higher.
-
 							*tags-option*
 The 'tags' option is a list of file names.  Each of these files is searched
 for the tag.  This can be used to use a different tags file than the default
@@ -484,7 +475,6 @@ ctags			As found on most Unix systems.  Only supports C.  Only
 exuberant ctags		This a very good one.  It works for C, C++, Java,
 			Fortran, Eiffel and others.  It can generate tags for
 			many items.  See http://ctags.sourceforge.net.
-etags			Connected to Emacs.  Supports many languages.
 JTags			For Java, in Java.  It can be found at
 			http://www.fleiner.com/jtags/.
 ptags.py		For Python, in Python.  Found in your Python source

runtime/doc/various.txt

@@ -330,7 +330,6 @@ N  *+dialog_con*	Support for |:confirm| with console dialog.
 N  *+dialog_con_gui*	Support for |:confirm| with GUI and console dialog.
 N  *+digraphs*		|digraphs| *E196*
    *+dnd*		Support for DnD into the "~ register |quote_~|.
-B  *+emacs_tags*	|emacs-tags| files
 N  *+eval*		expression evaluation |eval.txt|
 N  *+ex_extra*		Vim's extra Ex commands: |:center|, |:left|,
 			|:normal|, |:retab| and |:right|

runtime/doc/vim_diff.txt

@@ -31,6 +31,7 @@ these differences.
 - 'backspace' defaults to "indent,eol,start"
 - 'encoding' defaults to "utf-8"
 - 'nocompatible' is always set
+- 'tags' defaults to "./tags;,tags"
 - 'ttyfast' is always set
 - 'wildmenu' is set by default
 - 'wildmode' defaults to "list:longest,full"
@@ -131,6 +132,7 @@ Other commands:
 
 Other compile-time features:
   EBCDIC
+  Emacs tags support
 
 Nvim does not have a built-in GUI and hence the following aliases have been
 removed: gvim, gex, gview, rgvim, rgview

src/nvim/option.c

@@ -1491,7 +1491,7 @@ static vimoption_T
   {"tags",        "tag",  P_STRING|P_EXPAND|P_VI_DEF|P_COMMA|P_NODUP,
    (char_u *)&p_tags, PV_TAGS,
    {
-     (char_u *)"./tags,tags",
+     (char_u *)"./tags;,tags",
      (char_u *)0L
    } SCRIPTID_INIT},
   {"tagstack",    "tgst", P_BOOL|P_VI_DEF,