use imsg to handle ALL kinds of IPC in gmid. This simplifies and shorten the code, and makes everything more uniform too.

i.e. don't print scary messages with LOG_CRIT priority!

obviously msg[datalen] is an off-by-one

this fixes a bug introduced with the prefork mechanics: every server process shared the same socket, and this would cause a race condition when multiple server processes asked for a script cgi being executed. This gives each server process its own socket to talk to the executor, so the race cannot happen.

this is to let the regression suite compile