mail/php7[2-4]-imap: change client from cclient to panda-cclient

There are some reports about PHP crashes, when using php-imap
(especially with OpenSSL and TSL 1.3). All this problems went away,
when using the panda-cclient instead of the old cclient.

Therefore we make the panda-cclient the new default,
but still allow cclient in order to allow backwards compatibility.

Special thanks to bofh for figuring out the complex details and
the solution!

Special thanks to Jason for being patience and
helpful about so many months!

Reported by: Jason de Cordoba <jason@aventia.pw>
Reviewed by: bofh
Sponsored by: Bounce Experts