Implement a more robust remote connection detection (who)

First, we switch from `who am i` to `who -m` because this is the POSIX
way and macOS supports it as well.

Second, if the above fails, we fall back to using just `who` and grep
the line that represents our TTY. This works e.g. with busybox systems.

Finally, we use (extremely) simplified regular expressions for detecting
IPv4, IPv6 and hostname. This is a best-effort attempt at detecting the
remote connections when SSH_CONNECTION is not set.

pure.zsh

@@ -484,10 +484,26 @@ prompt_pure_state_setup() {
 	if [[ -z $ssh_connection ]] && (( $+commands[who] )); then
 		# When changing user on a remote system, the $SSH_CONNECTION
 		# environment variable can be lost, attempt detection via who.
-		# Ignore local X sessions displayed as (:1).
-		if who am i | grep -v '(:[0-9]*)$' | grep -q '(.*)$' &>/dev/null; then
+		local who_out
+		who_out=$(who -m 2>/dev/null)
+		if (( $? )); then
+			# Who am I not supported, fallback to plain who.
+			who_out=$(who 2>/dev/null | grep ${TTY#/dev/})
+		fi
+
+		local reIPv6='([a-f0-9:]+:+)+[a-f0-9]+'  # Simplified, but matches loopback as well (::1).
+		local reIPv4='([0-9]{1,3}\.){3}[0-9]+'   # Simplified, allows invalid ranges.
+		# Here we assume two non-consecutive periods represents a
+		# hostname. This matches foo.bar.baz, but not foo.bar.
+		local reHostname='([.][^. ]+){2}'
+
+		# Usually the remote address is surrounded by parenthesis, but
+		# not on all systems (e.g. busybox).
+		local -H MATCH MBEGIN MEND
+		if [[ $who_out =~ "\(?($reIPv4|$reIPv6|$reHostname)\)?\$" ]]; then
 			ssh_connection=true
 		fi
+		unset MATCH MBEGIN MEND
 	fi
 
 	# show username@host if logged in through SSH