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

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

Include gmid.h as first header in every file, as it then includes config.h (that defines _GNU_SOURCE for instance). Fix also a warning about unsigned vs signed const char pointers in openssl.

add/remove syscalls from the BPF filter and move sandbox() after libevent initialisation

what if we receive a SIGHUP right after unblock_signal (or during the whole block_signals...unblock_signals) but *before* the wait_sighup? Yeah.