Make auth-source-pass-search understand port lists

Make auth-source-pass-search understand port lists

For cases such as a typical IMAP Gnus setup, auto-source-pass-search
is passed a list of "port aliases" like (993 "imaps" "imap" "993"
"143") in hopes of finding a matching ~/.password-store entry.

This modification makes this library understand and unroll the port
list so that, i.e. "domain:993", "domain:imaps"", "domain:imap",
etc. are computed as potential suffixes.  Previously a nonsensical
string "domain:(993 imaps imap ...)" was returned.

* lisp/auth-source-pass.el
(auth-source-pass--generate-entry-suffixes): Allow PORT to be a list
of ports.