signal.c: move signalfd parsers to a separate file

* signalfd.c: New file.
* Makefile.am (strace_SOURCES): Add it.
* signal.c (do_signalfd, sys_signalfd, sys_signalfd4): Move to signalfd.c
diff --git a/Makefile.am b/Makefile.am
index 5837782..ff05a25 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -104,6 +104,7 @@
 	seccomp.c	\
 	sigaltstack.c	\
 	signal.c	\
+	signalfd.c	\
 	sigreturn.c	\
 	sock.c		\
 	socketutils.c	\