Following clue-stick applied by schwarze@, back out const-ness of regset

Following clue-stick applied by schwarze@, back out const-ness of regset
passed in to libmdoc and libman.

Fix mdoc.3 and man.3 EXAMPLE sections to include regset.

Add MDOC_SYNPRETTY flag cueing front-end to nicely format certain values
as if SEC_SYNOPSIS were the current section.