process.c: move clone, setns, unshare, and fork parsers to a separate file

* clone.c: New file.
* Makefile.am (strace_SOURCES): Add it.
* process.c: Move sys_clone, sys_setns, sys_unshare, sys_fork, and
related code to clone.c.
diff --git a/Makefile.am b/Makefile.am
index c4f3326..3671ce0 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -27,6 +27,7 @@
 	chdir.c		\
 	chmod.c		\
 	chown.c		\
+	clone.c		\
 	count.c		\
 	desc.c		\
 	dirent.c	\