Clean up warnings related to macros and nesting.
* Hierarchical naming of enum mandocerr items.
* Improve the wording to make it comprehensible.
* Mention the offending macro.
* Garbage collect one chunk of ancient, long unreachable code.
KNF.
better indentation
condition above makes this part of the check useless;
overlooked in the previous commit
u_int32_t's can't go negative
better indentation;  no functional change
Disentangle the MANDOCERR_CHILD message, which reported three
completely different things, into three distinct messages.
Also mention the macro names we are talking about.
Remove more unused cruft.

No objection from the usual suspects.
don't clear tmux session sockets in daily(8)'s tmp cleanup, from Rafael Zalamena
ok schwarze@