selftest: new module for testing baresip core

the selftest module is using the public API of the
Baresip core (in baresip.h) to verify the expected
behaviour and to test various protocol scenarios.

the module can be loaded with the 'module' directive
or 'module_tmp' directive:

    module_tmp        selftest.so

OR:

    module            selftest.so
play: fix warnings when compiling with OPT_SIZE=1