Allow setting TLS cipher suites through Sofia-SIP API

Previously there was no way to override the hard-coded cipher suite
specification of "ALL:!ADH:!LOW:!EXP:!MD5:@STRENGTH".

This commit does leave in place the hardcoded cipher spec for WebRTC
of "HIGH:!DSS:!aNULL@STRENGTH".
Support setting TLS cipher suites in mod_sofia

This adds a parameter tls-ciphers in which we can specify the TLS
cipher suite specification for OpenSSL.