Move sysctl parser to a separate file

* sysctl.c: New file.
* Makefile.am (strace_SOURCES): Add it.
* system.c: Move sys_sysctl and related code to sysctl.c.
diff --git a/Makefile.am b/Makefile.am
index 3c89725..9efebf1 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -52,6 +52,7 @@
 	strace.c	\
 	stream.c	\
 	syscall.c	\
+	sysctl.c	\
 	sysinfo.c	\
 	sysmips.c	\
 	system.c	\