Disable automatic following of redirect

[weili-jiang]

Currently all incoming 3xx responses are handled internally and followed through. I would like to be able to have some control over whether redirection (3xx) is automatically followed or not, based on what is in the 3xx message.

Would you accept a PR for a new API for re_sipsess.h as follows:

int sipsess_follow_redirects(struct sipsess *sess, bool enable);

This would set a bool flag on the session such that in invite_resp_handler can chose not to follow the redirect and just terminate the call with the 3xx message available to the client. I would call it immediately after sipsess_connect. Existing clients that do not call this new function will behave as before.

[sreimers]

A PR is welcome.