The function rew_sub() tries to rewind any all all kinds of blocks

The function rew_sub() tries to rewind any all all kinds of blocks
and elements under any and all circumstances, even handling some
bad block nesting now and then.  Little surprisingly, this ends up
in excessive complexity and has caused many bugs in the past.
Start to slowly disentangle this mess by replacing calls to rew_sub()
immediately following mdoc_head_alloc() by the much simpler rew_last().
Gets rid of the first two rew_sub() calls out of twenty.
No functional change.