Support accepting socket from system manager instead of managing own socket

Currently, the Keybase service insists on managing its socket itself; when passed an existing file, it attempts to delete the “stale” socket, and exits if that fails.

In the interest of managing Keybase with systemd (comments in run_keybase as well as in this issue tracker indicate that this is intended to be supported at some point), the service should also have an option to accept an open socket from the system manager (systemd). The best documentation for adding socket support to a program seems to be these two blog posts (systemd.socket(5) and systemd.service(5) focus more on the unit file, not on what the service has to do). In C, this seems to be the gist of it:

#include "sd-daemon.h"

int fd;
assert (sd_listen_fds(0) == 1);
fd = SD_LISTEN_FDS_START + 0;
assert (sd_is_socket(fd, AF_UNSPEC, SOCK_STREAM, 1));

There also appears to be a Go systemd binding, including an example for socket-based activation.

Note: macOS’ launchd apparently also supports socket-based activation. I’m not sure how similar the mechanism is.

Start with the socket-management comments in run_keybase, then read the linked socket-activation documentation and the Go systemd activation example. Determine where the Keybase service receives and validates its socket; done means it can accept an existing manager-provided socket without trying to remove it or exiting on failure.