utils.h: use inlined function instead of macro for belle_sip_logv to avoid tautological compilation error with clang

expose background_task api to application because it also needs it.

add a wrapper to getaddrinfo() to workaround a bug in android's libc not supporting AI_V4MAPPED flag.

split belle_sip_snprintf in 2 functions (belle_sip_snprintf and belle_sip_snprintf_valist) to enable usage with a va_list too

 - Use size_t instead of unsigned int for offset.
 - Add function to enable marshal check at runtime.
 - Enable marshal check in unit tests.

 - Use our own implementation of snprintf() called belle_sip_snprintf() to handle the various native implementations whose return values are not consistent.
 - Simplify error handling in marshalling.

moment.

document object memory management.