libunwind gives us a file and an address and usually a function name. Beyond
that, it's mostly guessing.
Fork off addr2line to resolve the addresses that libunwind gives us, if we
succeed we get a backtrace like this:

	Backtrace:
	0: litest_fail_comparison_int() (./test/litest.c:268)
	1: disable_button_scrolling() (./test/pointer.c:115)
	2: middlebutton_doubleclick() (./test/pointer.c:991)
	3: /lib64/libcheck.so.0 (srunner_run+0x7f5) [0x7f6c12d8c025]
	4: litest_run() (./test/litest.c:689)
	5: main() (./test/pointer.c:1280)
	6: /lib64/libc.so.6 (__libc_start_main+0xf0) [0x7f6c11a73790]
	7: ./test/test-pointer (_start+0x29) [0x403d99]
	8: ? (?+0x29) [0x29]

Note: I intentionally swapped function/file name in the output to make it
easier to spot which one is fully resolved and which one is the basic
libunwind output.

Signed-off-by: Peter Hutterer <peter.hutterer@who-t.net>