Add fsck_flags config option

On UFS with SU+J, sometimes fsck's default recovery from journal marks the
filesystem as clean but some errors remain.

With SU only, default fsck in preen mode sometimes thinks all errors have
been fixed but some still remain.

To address the issues above, this change adds a new config option:
fsck_flags. By default it's set to -p, but the user may change it to -y
or -f -y, for instance, to force a full fsck after a system crash.

Submitted by: jhibbits@ (original version)
Reviewed by: mckusick
Differential Revision: https://reviews.freebsd.org/D24087