journalctl: move access_check() to shared/

journalctl: move access_check() to shared/

The only functional change is that log_notice("No journal files were found.")
is not printed any more with --quiet. log_error("No journal files were opened
due to insufficient permissions.") is still printed.

I wasn't quite sure where to put this function, but shared/ seems to be the
right place and none of the existing files seem to fit too well.

v2: rename journal_access_check to journal_access_check_and_warn.