Put the code handling \} into a new function roff_cond_checkend()

Put the code handling \} into a new function roff_cond_checkend()
and call that function not only from both places where copies
existed - when processing text lines and when processing request/macro
lines in conditional block scope - but also when closing a macro
definition request, such that this construction works:

.if n \{.de macroname
macro content
.. \} ignored arguments
.macroname

This fixes a bug reported by John Gardner <gardnerjohng at gmail dot com>.

While here, avoid a confusing decrement of the line scope counter
in roffnode_cleanscope() for conditional blocks that do not have
line scope in the first place (no functional change for this part).
Also improve validation of an internal invariant in roff_cblock()
and polish some comments.