Add tst-sigaction.c to test BZ #23069

This simple test uses sigaction to define a signal handler. It then
uses sigaction again to fetch the information about the same signal
handler, and check that they are consistent. This is enough to detect
mismatches between struct kernel_sigaction and the kernel version of
struct sigaction, like in BZ #23069.

Changelog:
       * signal/tst-sigaction.c: New file to test BZ #23069.
       * signal/Makefile (tests): Fix indentation. Add tst-sigaction.