srtp: split srtp-module into 'srtp' and 'libsrtp'

libre v0.4.9 has a very fast and robust SRTP-stack and we want
to use it in baresip. however libsrtp is also working so we want
to keep the module using libsrtp around for a while.

two modules providing SDES-SRTP:

- libsrtp.so  -- SRTP using libsrtp
- srtp.so     -- Native SRTP-stack in libre