Remove #ifdefs TEMPDIRNAMES and add TEMPDIRNAMES for Windows

Vim does not define TEMPDIRNAMES for all systems, but it is defined for
all systems supported by Neovim.
Temporary directory names for Windows was obtained from GetTempPath()
function documentation at MSDN.
Additionally small renamings were performed.

src/nvim/os_unix_defs.h

@@ -204,9 +204,6 @@
   "~/.nvim,$VIM/vimfiles,$VIMRUNTIME,$VIM/vimfiles/after,~/.nvim/after"
 #  endif
 
-#  define TEMPDIRNAMES  "$TMPDIR", "/tmp", ".", "$HOME"
-#  define TEMPNAMELEN    256
-
 /* Special wildcards that need to be handled by the shell */
 #define SPECIAL_WILDCHAR    "`'{"