MFH (r309269): use malloc()ed buffers instead of stack buffers

MFC r308806

Speed up pw operations that edit /etc/group or /etc/passwd

r285050 fixed a bug in pw that could lead to /etc/passwd or /etc/group
corruption on power loss. However, it fixed it by opening those files with
O_SYNC, which is very slow, especially on ZFS. This change replaces O_SYNC
with appropriately placed fsync()s instead, which is much faster. Using a
ZFS tmpdir, the time to run pw's kyua tests drops from 245s to 35s.

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

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