Allow "update" option to be used in the fstab(5). Document possible use case.

Approved by:	mckusick
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D18586

sbin/mount/mount.c

@@ -361,6 +361,7 @@ main(int argc, char *argv[])
 				else
 					failok = 0;
 				if (!(init_flags & MNT_UPDATE) &&
+				    !hasopt(fs->fs_mntops, "update") &&
 				    ismounted(fs, mntbuf, mntsize))
 					continue;
 				options = update_options(options, fs->fs_mntops,

share/man/man5/fstab.5

@@ -190,6 +190,32 @@ see the
 .Xr mount 8
 manual page.
 .Pp
+If the option
+.Dq update
+is specified, it indicates that the status of an already mounted file
+system should be changed accordingly.
+This allows, for example, file systems mounted read-only to be upgraded
+read-write and vice-versa.
+By default, an entry corresponding to a file systems that is already
+mounted is going to be skipped over when processing
+.Nm ,
+unless it's a root file system, in which case logic similar to
+.Dq update
+is applied automatically.
+.Pp
+The
+.Dq update
+option is typically used in conjuction with two
+.Nm
+files.
+The first
+.Nm
+file is used to set up the initial set of file systems.
+The second
+.Nm
+file is then run to update the initial set of file systems and
+to add additional file systems.
+.Pp
 The type of the mount is extracted from the
 .Fa fs_mntops
 field and stored separately in the