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.

Reviewed by: allanjude, bapt, vangyzen, garga
Tested on pfSense by: garga
MFC after: 4 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D8319