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.

but why'd they called it program_invocation_short_name? They couldn't find a longer name?

this unlocks the full regress tests on arch, for instance, or every OS where we don't have strl*

This adds a check for setproctitle and for the (linux) prctl PR_SET_NAME. If setproctitle is not available, on linux we provide an implementation that use prctl (taken from tmux compat layer.)