Try to fix mailcap parsing again to respect Emacs defaults

* lisp/net/mailcap.el (mailcap--computed-mime-data): New variable.
(mailcap-parse-mailcaps): Don't delete Emacs-distributed fallback
values (bug#40247).
(mailcap-add-mailcap-entry): Extend to allow working on different
variables.
(mailcap-add): Store data in mailcap-user-mime-data, since it
should be heeded first.
1 job for master in 58 minutes and 48 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
failed #6798
test-all

00:58:48

 
Name Stage Failure
failed
test-all Test
Makefile:318: recipe for target 'check-doit' failed
make[2]: *** [check-doit] Error 1
make[2]: Leaving directory '/builds/emacs/emacs/test'
make[1]: *** [check-expensive] Error 2
Makefile:293: recipe for target 'check-expensive' failed
make[1]: Leaving directory '/builds/emacs/emacs/test'
Makefile:959: recipe for target 'check-expensive' failed
make: *** [check-expensive] Error 2
ERROR: Job failed: exit code 1