Fix man page markup for <cmdsynopsis> with multiple variants

Command synopses using <cmdsynopsis> with multiple variants previously used
<sbr> to break lines between variants.  The new man page toolchain introduced
in 9.0 makes a mess out of that, and that markup was probably wrong all along,
because <sbr> is supposed to break lines within a synopsis, not between them.
So fix that by using multiple <cmdsynopsis> elements inside <refsynopsisdiv>.

backpatched to 9.0