The lock for FILE is now .#FILE or .#-FILE.

The lock for FILE is now .#FILE or .#-FILE.

The old approach, which fell back on DIR/.#FILE.0 through
DIR/.#FILE.9, had race conditions that could not be easily fixed.
If DIR/.#FILE is a non-symlink file, Emacs now does not create a
lock file for DIR/FILE; that is, DIR/FILE is no longer partly
protected by a lock if DIR/.#FILE is a non-symlink file ("partly"
because the locking mechanism was never reliable in that case).
This patch fixes this and other bugs discovered by a code
inspection that was prompted by
<http://lists.gnu.org/archive/html/emacs-devel/2013-02/msg00531.html>.
Also, this patch switches to .#-FILE (not .#FILE) on MS-Windows,
to avoid interoperability problems between the MS-Windows and
non-MS-Windows implementations.  MS-Windows and non-MS-Windows
instances of Emacs now ignore each others' locks.
* etc/NEWS: Document this.
* src/filelock.c (defined_WINDOWSNT): New constant.
(MAKE_LOCK_NAME, fill_in_lock_file_name):
Don't create DIR/.#FILE.0 through DIR/.#FILE.9.  Instead, create
DIR/.#FILE symlinks on non-MS-Windows hosts, and DIR/.#-FILE
regular files on MS-Windows hosts.
(MAKE_LOCK_NAME, unlock_file, Ffile_locked_p):
Use SAFE_ALLOCA to avoid problems with long file names.
(MAX_LFINFO): Now a local constant, not a global macro.
(IS_LOCK_FILE): Remove.
(lock_file_1): Don't inspect errno if symlink call succeeds;
that's not portable.
(lock_file): Document that this function can return if lock
creation fails.

Fixes: debbugs:13807