This adds completion for the new -F, --field flag, but also uses this
option directly from journalctl to complete values for fields which
might be used as filters.
Use 'read -r' everywhere to consider backslashes as parts of the input line.
Single-quote the arguments to 'compgen -W' to avoid immediate expansion.
compgen itself will expand the argument.
Fixes a possible reason for "Failed to issue method call: Unknown unit"
after requesting completion.
https://bugzilla.redhat.com/show_bug.cgi?id=814966