pjsip: Increase maximum number of usable ciphers & other cleanups

* Increase maximum number of ciphers from 100 to 256 (or whatever
  PJ_SSL_SOCK_MAX_CIPHERS is #define'd to)

* Simplify logic in cipher_name_to_id()

* Make signed/unsigned comparison consistent

Re: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=897412

Reported by: Ondřej Holas

Change-Id: Iea620f03915a1b873e79743154255c3148a514e7