Fixes composer dependencies: Package phpunit/phpunit-mock-objects is abandoned
We cannot support v8 yet because of errors like:
Declaration of MailFunc::setUp() must be compatible with PHPUnit\Framework\TestCase::setUp(): void
It would require dropping PHP < 7.1 support.
* krb_authentication: remove default principal
krb_authentication_context must be set to the right value to make GSSAPI work, we should not have any default for it outside config files, it could confuse users. We already handle empty/bad context in rcube_imap_generic.php, Net_Sieve, Net_SMTP.
* krb_authentication: config.php.dist fix
GSSAPI is better for searching, since GSS API is not used widely.
host.fqdn@REALM.NAME is actually never used without service-prefix, on the other side it may be valid to use 'computername$' or 'username' with Samba/Active Directory, the common name for all these - 'principal'.
* krb_authentication: raise error on empty context
