Following revision r295924, the changes to a db file should be fsynced
before the file is closed. Consequently, it shouldn't be necessary to
open the file with O_SYNC any more.

This improves the performance of building large .db files for large
password files a lot and should resolve this problem:

https://forums.freebsd.org/threads/10-2-pre-pwd_mkdb-slow-on-larger-master-passwd.52700/

Differential Revision: https://reviews.freebsd.org/D5186
Reviewed by: garga, vangyzen, bapt, se
MFC after: 1 week