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

* 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