file.c: move link, linkat, unlinkat, and symlinkat parsers to a separate file

* link.c: New file.
* Makefile.am (strace_SOURCES): Add it.
* file.c: Move sys_link, sys_linkat, sys_unlinkat, sys_symlinkat, and
related code to link.c.
diff --git a/Makefile.am b/Makefile.am
index c8a6871..e78e1fe 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -40,6 +40,7 @@
 	kexec.c		\
 	keyctl.c	\
 	ldt.c		\
+	link.c		\
 	loop.c		\
 	mem.c		\
 	mknod.c		\