New options 'supported', 'require' and 'accept'

* Now SipApp's configuration allows to setup the contents headers "Supported" and "Accept" should have when options 'make_supported' and 'make_accept' are used.

* It is possible to use option 'require' in nksip_uac to force the generation of a "Require" header

* It is possible to use option 'accept' along with 'make_accept' in nksip_uac to change the contents of "Accept" header.

* When working as an UAC, NkSIP will check any incoming request, and in case it has a "Require" header with a token not recognized. Now tokens defined in 'supported' options are also accepted.