fix(:bcd): do not "inherit" buffer-local dir

Problem:
Buffer-local CWD (:bcd) is "sticky", similar to window-local CWD (:lcd).
But this contradicts one of its main benefits: per-buffer "project root"
for LSP, OSC7.

Other problems:
- A buffer created with :edit/:enew/:new silently inherits b_localdir
  (and b_prevdir) from the previous buffer.
- curbuf_reusable() refuses to recycle a scratch buffer that has
  `b_localdir`.
- After :new/:vnew/:tabnew the CWD sticks to previous buffer's
  `b_localdir` even though the new curbuf has none, so :new is not
  equivalent to ":split | enew", and getcwd() disagrees with
  haslocaldir().
- Requires "which buffer spawned this buffer" semantics that no other
  buffer-local state has.

Solution:
Drop sticky/inherit behavior of buffer-local CWD (:bcd).

- do_ecmd: always apply the new curbuf's dir (`fix_current_dir`), like
  `do_autochdir` already does. :tabnew from a :bcd buffer now reverts to
  global CWD (and fires DirChanged), same as :tabnew from a :lcd window.
- curbuf_reusable(): recycling a scratch buffer frees its b_localdir.

To get sticky/inherit behavior of CWD, use `:lcd`.

src/nvim/options.lua

@@ -1093,7 +1093,7 @@ local options = {
         doc = [[on on Unix, off on Windows]],
       },
       desc = [=[
-        When on, |:cd|, |:tcd| and |:lcd| without an argument changes the
+        When on, |:cd|, |:tcd|, |:lcd| and |:bcd| without an argument changes the
         current working directory to the |$HOME| directory like in Unix.
         When off, those commands just print the current directory name.
       ]=],
@@ -1113,7 +1113,7 @@ local options = {
       deny_duplicates = true,
       desc = [=[
         This is a list of directories which will be searched when using the
-        |:cd|, |:tcd| and |:lcd| commands, provided that the directory being
+        |:cd|, |:tcd|, |:lcd| and |:bcd| commands, provided that the directory being
         searched for has a relative path, not an absolute part starting with
         "/", "./" or "../", the 'cdpath' option is not used then.
         The 'cdpath' option's value has the same form and semantics as
@@ -2119,7 +2119,7 @@ local options = {
         		following occurrence.
         							*cpo-~*
         	~	When included, don't resolve symbolic links when
-        		changing directory with |:cd|, |:lcd|, or |:tcd|.
+        		changing directory with |:cd|, |:tcd|, |:lcd|, or |:bcd|.
         		This preserves the symbolic link path in buffer names
         		and when displaying the current directory.  When
         		excluded (default), symbolic links are resolved to