EUDC: Add ecomplete and mailabbrev backends

EUDC: Add ecomplete and mailabbrev backends

* doc/misc/eudc.texi (Overview): Add ecomplete and mailabbrev
nodes.
(ecomplete, mailabbrev): New nodes.
(Installation): Add ecomplete and mailabbrev nodes.
(LDAP Configuration): Use code formatting instead of quotes.
(macOS Contacts Configuration): Likewise.
(ecomplete Configuration): New node.
(mailabbrev Configuration): Likewise.
* etc/NEWS (EUDC): Mention ecomplete and mailabbrev backends,
mention eudc-server-hotlist default change.
* lisp/net/eudc-vars.el (eudc-known-protocols): Add ecomplete and
mailabbrev.
(eudc-server-hotlist): Add entries for ecomplete and mailabbrev.
* lisp/net/eudcb-ecomplete.el: New EUDC backend file.
* lisp/net/eudcb-mailabbrev.el: Likewise.
* test/lisp/net/eudc-resources/ecompleterc,
test/lisp/net/eudc-resources/mailrc: New eudc-tests resource
files.
* test/lisp/net/eudc-tests.el (eudc-test-rfc5322-quote-phrase)
(eudc-test-make-address, eudcb-ecomplete, eudcb-mailabbrev): New
test cases.