I really want to get rid of the `executor' process hack for CGI scripts and its escalation to allow fastcgi and proxying to work on non-OpenBSD. This drops the CGI support and the `executor' process entirely and is the first step towards gmid 2.0. It also allows to have more secure defaults. On non-OpenBSD systems this means that the sandbox will be deactivated as soon as fastcgi or proxying are used: you can't open sockets under FreeBSD' capsicum(4) and I don't want to go thru the pain of making it work under linux' seccomp/landlock. Patches are always welcome however. For folks using CGI scripts (hey, I'm one of you!) not all hope is lost: fcgiwrap or OpenBSD' slowcgi(8) are ways to run CGI scripts as they were FastCGI applications. fixes for the documentation and to the non-OpenBSD sandboxes will follow.

steal more (good) stuff from mandoc-portable :)

While here, move the SRCS variable to the configure and add the -includes in Makefile.local; it de-clutters the Makefile a bit.