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/.gitignore b/tests/.gitignore
index c701874..a37c052 100644
--- a/tests/.gitignore
+++ b/tests/.gitignore
@@ -84,6 +84,7 @@
 rt_sigpending
 rt_sigprocmask
 rt_sigqueueinfo
+rt_sigtimedwait
 sched_xetaffinity
 sched_xetattr
 scm_rights