Make "-S" a default option for mountd.

After a discussion on freebsd-fs@ there seemed to be a consensus that
the "-S" option for mountd should become the default.
Since the only known issue w.r.t. using "-S" was fixed by r299201,
this commit adds "-S" to the default mountd_flags.

Discussed on: freebsd-fs
PR: 9619, 131342, 206855
MFC after: 2 weeks
Relnotes: yes