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.