Make doc-string of window-total-size more self-contained.

* window.el (window-total-size): Make doc-string more
self-contained.
Don't prevent random file systems from being unmounted.

This fix relies on having the 'fchdir' function, and on having
"." be searchable (or at least readable, on platforms lacking O_SEARCH),
but that's good enough to handle the vast majority of cases and the
remaining folks can just live with the annoyance of file systems
that occasionally can't be unmounted.
* configure.ac (fchdir): New function to check for.
* lib/save-cwd.c: Copy from gnulib, except omit the part that
allocates memory, since that can cause problems in Emacs.
* lib/save-cwd.h: Copy from gnulib.
Simplify previous patch.
Revert 2013-01-31 change that decides coding system before backing up

It causes a more serious problem than the one it solves.
This closes bug#18141, and reopens bug#13522.

* lisp/files.el (basic-save-buffer-2): Revert 2013-01-31 change.

* src/fileio.c: Revert 2013-01-31 change.
(choose_write_coding_system): No longer callable from Lisp.
Move last piece back here from Fwrite_region.
(Fwrite_region, syms_of_fileio): Update for above changes.

* test/automated/data/files-bug18141.el.gz: New file.
* test/automated/files.el (files-test-bug-18141-file):
New variable and test.
* lisp/subr.el (with-output-to-temp-buffer): Doc fix; from elisp manual.
* src/gmalloc.c (_malloc_mutex, _aligned_blocks_mutex) [CYGWIN]: Use ERRORCHECK mutexes.  (Bug#18222)
* speedbar.el (speedbar-generic-list-tag-p): Allow special
elements from imenu.
Bump version number to 24.3.93
Regenerate etc/AUTHORS
Regenerate ldefs-boot.el