isync Merge Request #2: use SNI when connecting with SSL (merged)

imap.gmail.com doesn't accept connections without SNI anymore. Without
this extension, it returns a self-signed certificate and mbsync is
unable to complete:

$ openssl s_client -connect imap.gmail.com:993 -noservername
CONNECTED(00000005)
depth=0 OU = "No SNI provided; please fix your client.", CN = invalid2.invalid
verify error:num=18:self signed certificate
verify return:1
depth=0 OU = "No SNI provided; please fix your client.", CN = invalid2.invalid
verify return:1
---
Certificate chain
 0 s:OU = "No SNI provided; please fix your client.", CN = invalid2.invalid
   i:OU = "No SNI provided; please fix your client.", CN = invalid2.invalid

This commit configure the SSL connection to transmit the hostname
through SNI. This has been tested with both GMail (which requires SNI)
and Fastmail (which doesn't require SNI).

[Vincent Bernat]

For some reason, this only happens with OpenSSL 1.1.1.

[Oswald Buddenhagen]

i merged a derived patch.