Up to version 1.22.4, groff_mdoc(7) only considered the first word

Up to version 1.22.4, groff_mdoc(7) only considered the first word
when comparing section headers.  For example, ".Sh SEE ELSEWHERE"
and ".Sh SEE Em ALSO" were considered instances of a SEE ALSO
section.  In groff-current, exact matches with no sub-macros are
required.  Adjust mandoc behaviour.

While here, also fix a very minor mandoc bug, even though no
detrimental effect of the bug on formatting is known.  While using
sub-macros in the .Sh HEAD is bad style, the parsers accept it, so
setting the section attribute on the HEAD needs to act recursively.