First move towards landlock support (#3). The shim is needed until
libc provides the proper wrappers for the landlock APIs; I hope it
doesn't take too long, but landlock was merged back in May and are
still missing.
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.)