Reduce fstab(5)/mount(8) confusion by changing the man pages to say "ro"

instead of "rdonly".  "rdonly" works for mount(8) and mount_std(8) but
not from /etc/fstab, whereas "ro" works for all mount_*(8) commands.

sbin/mount/mount.8

@@ -180,7 +180,7 @@ It is set automatically when the user does not have super-user privileges.
 .It Cm nosymfollow
 Do not follow symlinks
 on the mounted file system.
-.It Cm rdonly
+.It Cm ro
 The same as
 .Fl r ;
 mount the file system read-only (even the super-user may not write it).
@@ -327,7 +327,7 @@ option.
 The file system is to be mounted read-only.
 Mount the file system read-only (even the super-user may not write it).
 The same as the
-.Cm rdonly
+.Cm ro
 argument to the
 .Fl o
 option.

sbin/mount_std/mount_std.8

@@ -71,8 +71,7 @@ A ``standard'' file system is one which:
 accepts only the standard
 .Fl o
 options
-.Dq ro
-.Pq Dq rdonly ,
+.Dq ro ,
 .Dq rw ,
 .Dq nodev ,
 .Dq noexec ,