bash-completion: use list-unit-files to get "all" units

I think that it is better to return good results slightly more slowly,
than partial quickly. Also reading from disk seems fast enough. Even
the delay on first try with completely cold cache is acceptable.

This is just for bash, 'cause zsh was already doing this.

https://bugzilla.redhat.com/show_bug.cgi?id=790768

shell-completion/bash/systemctl.in

@@ -51,11 +51,13 @@ __filter_units_by_property () {
         done
 }
 
-__get_all_units      () { __systemctl $1 list-units --all \
+__get_all_units      () { { __systemctl $1 list-unit-files; __systemctl $1 list-units --all; } \
         | { while read -r a b; do echo " $a"; done; }; }
 __get_active_units   () { __systemctl $1 list-units       \
         | { while read -r a b; do echo " $a"; done; }; }
-__get_startable_units () { __systemctl $1 list-units --all -t service,timer,socket,mount,automount,path,snapshot,swap \
+__get_startable_units () { {
+        __systemctl $1 list-units --all -t service,timer,socket,mount,automount,path,snapshot,swap
+        __systemctl $1 list-unit-files -t service,timer,socket,mount,automount,path,snapshot,swap; } \
         | { while read -r a b c d; do [[ $c == "inactive" || $c == "failed" ]] && echo " $a"; done; }; }
 __get_failed_units   () { __systemctl $1 list-units       \
         | { while read -r a b c d; do [[ $c == "failed"   ]] && echo " $a"; done; }; }