linux/bsd/mac: Use pkill to stop remote instance over SSH
Previously, the PIDs of any running instances of the game on the remote
device were found with `pgrep`, whose output was passed as parameters to
`kill`. The problem with doing this is that passing zero arguments to
`kill` (which happens when no instances of the game are running
remotely) is an error: it shows the command usage, and exits with status
2 indicating a command-line syntax error:
$ kill
kill: usage: kill [-s sigspec | -n signum | -sigspec] pid | jobspec ... or kill -l [sigspec]
$ echo $?
2
As far as I can tell, all systems that have a `pgrep` command also have
a `pkill` command which accepts (a superset of) the same parameters as
`pgrep` and kills the matched processes instead of listing them on
STDOUT.
In the case where no processes match, `pkill` exits with status 1; but
does so silently.
Invoke `pkill` rather than `pgrep` + `kill`.
Will Thompson