instaweb: support mod_cgid for apache2

instaweb: support mod_cgid for apache2

Some people have mod_cgid instead of mod_cgi, most likely as a result of
choosing a threaded MPM.

In cases where the user has both modules, mod_cgi will be preferred in
order to maintain a simpler setup.

This patch also causes instaweb to print a message and die in cases
where there is no module that instaweb knows how to handle.

Signed-off-by: Mark Rada <marada@uwaterloo.ca>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>