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.
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...)