Element next-line scopes may nest, so man_breakscope() may have to

Element next-line scopes may nest, so man_breakscope() may have to
break multiple element next-line scopes at the same time, similar to
what man_descope() already does for unconditional rewinding.

This fixes an assertion failure that tb@ found with afl(1), caused
by .SH .I .I .BI and similar sequences of macros without arguments.