Sync to bsd.lv; in particular, pull in lots of bug fixes.

Sync to bsd.lv; in particular, pull in lots of bug fixes.
new features:
* support the .in macro in man(7)
* support minimal PDF output
* support .Sm in mdoc(7) HTML output
* support .Vb and .nf in man(7) HTML output
* complete the mdoc(7) manual
bug fixes:
* do not let mdoc(7) .Pp produce a newline before/after .Sh; reported by jmc@
* avoid double blank lines related to man(7) .sp and .br
* let man(7) .nf and .fi flush the line; reported by jsg@ and naddy@
* let "\ " produce a non-breaking space; reported by deraadt@
* discard \m colour escape sequences; reported by J.C. Roberts
* map undefined 1-character-escapes to the literal character itself
maintenance:
* express mdoc(7) arguments in terms of an enum for additional type-safety
* simplify mandoc_special() and a2roffdeco()
* use strcspn in term_word() in place of a manual loop
* minor optimisations in the -Tps and -Thtml formatting frontends