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.

on fedora 33 the BIGNUM type is opaque. Allocate always to avoid headaches.

keep mark_nonblock in utils.c, as otherwise the build for the regress suite will fail (mark_nonblock needs fatal which is in gmid.c, and we can't link gmid.o with the regress suite...)