Account based configuration to specify if "c=0.0.0.0" method should be used when putting call on hold (thanks Marcus Froeschl for the suggestion)

[bennylp]

PJSIP used to use c=0.0.0.0 method to signal call hold to remote party, but we changed that in #563 because this is no longer recommended by RFC 3264. According to this RFC, we should signal call hold by using sendonly/recvonly/inactive attribute, which is exactly what we do now.

However, some other peer still uses the old method to convey call hold, and when talking to this peer, we could never put the call on hold.

An account based configuration to control how call hold should be conveyed would be beneficial to promote interoperability with more software.

[bennylp]

(In [3330]) Closed #1142 (Account based configuration to specify if "c=0.0.0.0" method should be used when putting call on hold):

• use PJSUA_CALL_HOLD_TYPE_DEFAULT to specify default global call hold type
• use pjsua_acc_config.call_hold_type to specify call hold type for the account
• call hold type can also be set on per call basis by changing the call_hold_type in the call structure (requires inclusion of <pjsua-lib/pjsua_internal.h>