Automatically handle 423 (Interval Too Brief) response in SIP registration (thanks Tomáš Valenta for the suggestion)

[bennylp]

Currently 423 response will cause registration to terminate, and worse there is no good way for application to query the desired interval indicated by the registrar in the response¹⁾.

It would be better if the library handles this automatically.

^{1) Actually application can, with registering a pjsip module and peek the REGISTER response, but this is not a clean approach for this.}

[bennylp]

Done in r3105:

• implemented in sip_reg.c instead of in PJSUA-LIB, so that the functionality can be reused by non-PJSUA-LIB applications
• also added several Python test scripts