sip_session: internal method rename (_ps -> _prompt)

darcs-hash:20090330065942-b2873-aca17d46123749b041c6f2292ba059551341c65a
sip_session: add :dtmf command (:dtmf 0000)

darcs-hash:20090330070610-b2873-0e841059cedb0e48a9d2f9ba74b27bfd2b7c894e