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

split usage string into two lines

give each server process its own socket for the executor 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.

move log_init & vars to gmid.c, retain logger_main in log.c this is to let the regression suite compile

moving logging to its own process

fix various compilation errors 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 newline after usage

fix seccomp for the new event loop add/remove syscalls from the BPF filter and move sandbox() after libevent initialisation

improve logs management

added prefork option

add the ``entrypoint'' option

added ``block return'' and ``strip'' options

missing argument for LOGI

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

reload configuration on SIGHUP