nih/logging.c: use our own __nih_abort_msg rather than the (e)glibc symbol

1) __abort_msg is not available in all libc's
2) when it is available, it creates a tight dependancy between libnih and (e)glibc, requiring libnih to be recompiled against each (e)glibc upload
3) equivalent functionality can be provided by our own implementation on abort_msg.
4) apport in ubuntu has been modifyied to look up crash message from __nih_abort_msg in additional to __abort_msg, thus decoupling (e)glibc from libnih

This patch creates our own __nih_abort_msg. There are other alternatives that could be implemented (e.g. add a conditional to offer using __abort_msg (if available) and select/fallback to __nih_abort_msg. Let me know your thoughts on this.

* nih/logging.c: Use our own __nih_abort_msg rather than the (e)glibc private symbol __abort_msg to avoid upgrade issues (LP: #997359).
* nih/tests/test_logging.c: Update tests for __nih_abort_msg.

Bug-Ubuntu: https://bugs.launchpad.net/bugs/997359