In set and unset commands, the <option> must not be quoted. In the
sample protocol.user.mailto.unix setting, the internal quotes were
using some manpage syntax, resulting in incorrect HTML output.
This simplifies help2doc since it now just have to do one single thing with
output of either --config-help or --long-help. The new manpage is not as
fancy wrt. indentation and sub titles but it is more uniform and we get a
HTML version.
Generate options-{config,command}.txt with help2doc. Only conditionally
check for API header files, to speed up execusion.
