tm: flag core to use forced socket when uac socket is set

[mslehto]

Pre-Submission Checklist

• Commit message has the format required by CONTRIBUTING guide
• Commits are split per component (core, individual modules, libs, utils, ...)
• Each component has a single commit (if not, squash them into one commit)
• No commits to README files for modules (changes must be done to docbook files
  in doc/ subfolder, the README file is autogenerated)

Type Of Change

• Small bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds new functionality)
• Breaking change (fix or feature that would change existing functionality)

Checklist:

• PR should be backported to stable branches
• Tested changes locally
• Related to issue #XXXX (replace XXXX with an open issue number)

Description

While testing Kamailio instance with multiple interfaces I noticed that dispatcher probing does not use the socket defined on dispatcher list_file.
By setting flag SND_F_FORCE_SOCKET, in tm module, dispatcher probing or tm t_uac_send() is sourced as per socket attribute.

Tested on FreeBSD 11.1 and Ubuntu 16.04.

[miconda]

Thanks!