journalctl: add --system/--user flags

--user basically gives messages from your own systemd --user services.
--system basically gives messages from PID 1, kernel, and --system
services. Those two options are not exahustive, because a priviledged
user might be able to see messages from other users, and they will not
be shown with either or both of those flags.

shell-completion/bash/journalctl

@@ -38,6 +38,7 @@ _journalctl() {
         local field_vals= cur=${COMP_WORDS[COMP_CWORD]} prev=${COMP_WORDS[COMP_CWORD-1]}
         local -A OPTS=(
                 [STANDALONE]='-a --all --full
+                              --system --user
                               -b --this-boot --disk-usage -f --follow --header
                               -h --help -l --local --new-id128 -m --merge --no-pager
                               --no-tail -q --quiet --setup-keys --this-boot --verify