Document the mntopts(3) functions.

The mntopts(3) functions support operations associated with a mount
point. The main purpose of this commit is to document the mntopts(3)
functions that now appear in 18 utilities in the base system. See
mntopts(3) for the documentation details.

The getmntopts() function appeared in 4.4BSD. The build_iovec(),
build_iovec_argf(), free_iovec(), checkpath(), and rmslashes()
functions were added with nmount(8) in FreeBSD 5.0. The getmntpoint()
and chkdoreload() functions are being added in this commit.

These functions should be in a library but for historic reasons are
in a file in the sources for the mount(8) program. Thus, to access
them the following lines need to be added to the Makefile of the
program wanting to use them:

SRCS+= getmntopts.c
MOUNT= ${SRCTOP}/sbin/mount
CFLAGS+= -I${MOUNT}
.PATH: ${MOUNT}

Once these changes have been MFC'ed to 13 they may be made into
a library.

Reviewed by: kib, gbe
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37907