Enhance parser of struct sigevent

Enhance parser of struct sigevent

* print_sigevent.c (print_sigevent): Always print names
of struct_sigevent fields along with their values.
Print signal name of sigev_signo for those sigev_notify values
that require valid signal numbers.
* tests/timer_create.c: New file.
* tests/timer_create.test: New test.
* tests/Makefile.am (check_PROGRAMS): Add timer_create.
(TESTS): Add timer_create.test.
* tests/.gitignore: Add timer_create.