MFC r285050, r285053, r285059:

MFC r285050, r285053, r285059:

When passwd or group information is changed (by pw, vipw, chpass, ...)
temporary file is created and then a rename() call move it to official file.
This operation didn't have any check to make sure data was written to disk
and if a power cycle happens system could end up with a 0 length passwd
or group database.

There is a pfSense bug with more information about it:

https://redmine.pfsense.org/issues/4523

The following changes were made to protect passwd and group operations:

* lib/libutil/gr_util.c:
 - Replace mkstemp() by mkostemp() with O_SYNC flag to create temp file
 - After rename(), fsync() call on directory for faster result

* lib/libutil/pw_util.c
 - Replace mkstemp() by mkostemp() with O_SYNC flag to create temp file

* usr.sbin/pwd_mkdb/pwd_mkdb.c
 - Added O_SYNC flag on dbopen() calls
 - After rename(), fsync() call on directory for faster result

* lib/libutil/pw_util.3
 - pw_lock() returns a file descriptor to master password file on success

Differential Revision:	https://reviews.freebsd.org/D2978
Approved by:	re (kib), bapt (implicit agreed)
Sponsored by:	Netgate

git-svn-id: svn://svn.freebsd.org/base/stable/10@285205 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f