file.c: move readlink and readlinkat parsers to a separate file

* readlink.c: New file.
* Makefile.am (strace_SOURCES): Add it.
* file.c (decode_readlink, sys_readlink, sys_readlinkat): Move
to readlink.c.
diff --git a/Makefile.am b/Makefile.am
index 324be2b..c8a6871 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -53,6 +53,7 @@
 	process.c	\
 	ptp.c		\
 	quota.c		\
+	readlink.c	\
 	reboot.c	\
 	renameat.c	\
 	resource.c	\