Extract user from host when searching for entries in auth-source-pass

Extract user from host when searching for entries in auth-source-pass

* lisp/auth-source-pass.el (auth-source-pass--user): New function.
(auth-source-pass--find-match): Use it.  When the user is not
explicitly specified and no entry is found, extract the user from
the host and then search again.  (Bug#29045)
* test/lisp/auth-source-pass-tests.el
(auth-source-pass-find-match-matching-extracting-user-from-host):
Add a new test case.

Copyright-paperwork-exempt: yes