Improve __fish_print_service_names

- Reuse systemd completion functions
- Remove a fork
- Fix #2715 - FreeBSD
- Comments

share/functions/__fish_print_service_names.fish

@@ -1,9 +1,13 @@
 function __fish_print_service_names -d 'All services known to the system'
-    if test -d /run/systemd/system
-        command systemctl list-units  -t service | cut -d ' ' -f 1 | grep '\.service$' | sed -e 's/\.service$//'
-    else if type -f rc-service 2> /dev/null
+    if test -d /run/systemd/system # Systemd systems
+		# For the `service` command, this needs to be without the type suffix
+		# on Debian at least
+		__fish_systemctl_services | string replace -r '.service$' ''
+    else if type -f rc-service ^/dev/null # OpenRC (Gentoo)
         command rc-service -l
-    else
-        command ls /etc/init.d
+    else if test -d /etc/init.d # SysV on Debian and other linuxen
+		string replace '/etc/init.d/' '' -- /etc/init.d/*
+	else # Freebsd
+		command service -l
     end
 end