Move mntopts(3) suppport into libutil

Many programs use this functionality so it should be available centrally
rather than compiled into each program independently. This has the
modest downside of adding libutil dependencies to some mount_<foo>
programs, but many (plus mount(8)) already have those depndencies so
this shouldn't be a major imposition.

In commit 906c312bbf74, Kirk said this could be done once the change was
merged to FreeBSD 13. That happened in commit 668dfa016698 in 2023.

While here perform related tidying including removing SRCS where the
only entry is ${PROG}.c rendering it unnecessary.

Reviewed by: olce, dim, emaste
Differential Revision: https://reviews.freebsd.org/D49951