log: support strings as level selectors in WIREPLUMBER_DEBUG

This allows for WIREPLUMBER_DEBUG="D" which avoids users having to
remember which numerical value the various levels have. The strings to
select are the ones printed by the logger, i.e. UECWMIDT.
7 jobs for wip/logging in 2 minutes and 6 seconds (queued for 4 seconds)
Status Name Job ID Coverage
  Container
passed container_fedora #15955599

00:00:17

passed container_ubuntu #15955600

00:00:16

 
  Build
passed build_on_fedora_no_docs #15955602

00:00:42

passed build_on_fedora_with_docs #15955601

00:00:51

passed build_on_ubuntu_no_gir #15955604

00:00:46

passed build_on_ubuntu_with_gir #15955603

00:00:48

 
  Analysis
passed shellcheck #15955605

00:00:57