handler: make 'protocol' always specified as a single bit

This makes the findprotocol() function work as intended so that libcurl
can properly be restricted to not support HTTP while still supporting
HTTPS - since the HTTPS handler previously set both the HTTP and HTTPS
bits in the protocol field.

This fixes --proto and --proto-redir for most SSL protocols.

This is done by adding a few new convenience defines that groups HTTP
and HTTPS, FTP and FTPS etc that should then be used when the code wants
to check for both protocols at once. PROTO_FAMILY_[protocol] style.

Bug: https://github.com/bagder/curl/pull/97
Reported-by: drizzt

lib/imap.c

@@ -155,7 +155,7 @@ const struct Curl_handler Curl_handler_imaps = {
   imap_disconnect,                  /* disconnect */
   ZERO_NULL,                        /* readwrite */
   PORT_IMAPS,                       /* defport */
-  CURLPROTO_IMAP | CURLPROTO_IMAPS, /* protocol */
+  CURLPROTO_IMAPS,                  /* protocol */
   PROTOPT_CLOSEACTION | PROTOPT_SSL |
   PROTOPT_NEEDSPWD                  /* flags */
 };