Print rt_sigtimedwait return value as a signal name

* signal.c (SYS_FUNC(rt_sigtimedwait)): Decode return value.
* tests/rt_sigtimedwait.c: New file.
* tests/rt_sigtimedwait.test: New test.
* tests/.gitignore: Add rt_sigtimedwait.
* tests/Makefile.am (check_PROGRAMS): Likewise.
(TESTS): Add rt_sigtimedwait.test.
diff --git a/tests/Makefile.am b/tests/Makefile.am
index 7030a29..de9bfb1 100644
--- a/tests/Makefile.am
+++ b/tests/Makefile.am
@@ -132,6 +132,7 @@
 	rt_sigpending \
 	rt_sigprocmask \
 	rt_sigqueueinfo \
+	rt_sigtimedwait \
 	sched_xetaffinity \
 	sched_xetattr \
 	scm_rights \
@@ -284,6 +285,7 @@
 	rt_sigpending.test \
 	rt_sigprocmask.test \
 	rt_sigqueueinfo.test \
+	rt_sigtimedwait.test \
 	sched_xetaffinity.test \
 	sched_xetattr.test \
 	scm_rights-fd.test \