dirmngr: Help detection of bad keyserver configurations.

* dirmngr/server.c (count_colons): New.
(cmd_keyserver): Print a warning status in some cases.

Because those colon delimited specifications (aka opaque) may be used
for other protocols than ldap we can't easily reject them at the
parsing stage. To help the support teams, we now print a warning if
we notice no ldap and more than 3 colons in the uri:

S KEYSERVER ldap.foo.org::user:password::plain
S WARNING keyserver_config 0 The "ldap:" prefix might \
                             be missing before the host name

Related to

• GnuPG-bug-id: T7730