Have pwd_mkdb lock the source file while rebuilding the database. When
called by programs such as vipw, the source file is a temporary file and
this does not conflict with the lock on /etc/master.passwd already held
by vipw. When run manually, however, master.passwd is typically specified
as the argument and the locking prevents other programs from messing with
master.passwd during the database rebuild.
Also pwd_mkdb uses a blocking exclusive lock as it may be called from
a script. The -N option was added to cause pwd_mkdb to get the lock
non-blocking and exit with an error if the attempt fails, again useful
for scripts.
