Use .In instead of .Ar Pa for header files.

Improve RETURN VALUES section.

Mark up NULL.

Use Aq instead of \*[Lt]\*[Gt].

Use
.In header.h
instead of
.Fd #include \*[Lt]header.h\*[Gt]
Much easier to read and write, and supported by groff for ages.
Okayed by ross.

Normalize Dd.

Generate <>& symbolically.

Whitespace nits.

Add man pages for libform.