file.c: move mknod, mknodat, and xmknod parsers to a separate file

* mknod.c: New file.
* Makefile.am (strace_SOURCES): Add it.
* file.c: Move sys_mknod, sys_mknodat, sys_xmknod, and related code
to mknod.c.
diff --git a/Makefile.am b/Makefile.am
index bae7d43..0516344 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -40,6 +40,7 @@
 	ldt.c		\
 	loop.c		\
 	mem.c		\
+	mknod.c		\
 	mount.c		\
 	mtd.c		\
 	net.c		\