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

shell-completion/systemd-zsh-completion.zsh

@@ -71,6 +71,8 @@ _ctls()
                 '--since=[Start showing entries newer or of the specified date]:YYYY-MM-DD HH\:MM\:SS' \
                 '--until=[Stop showing entries older or of the specified date]:YYYY-MM-DD HH\:MM\:SS' \
                 {-c,--cursor=}'[Start showing entries from specified cursor]:cursors:_journal_fields __CURSORS' \
+                '--system[Show system and kernel messages]' \
+                '--user[Show messages from user services]' \
                 {-b,--this-boot}'[Show data only from current boot]' \
                 {-u,--unit=}'[Show data only from the specified unit]:units:_journal_fields _SYSTEMD_UNIT' \
                 '--user-unit[Show data only from the specified user session unit]:units:_journal_fields _SYSTEMD_USER_UNIT' \