add link_cow feature

When enabled if a regular file is opened which has a link count > 1 it will
copy the file to a temporary file and rename over the original. Effectively
breaking the link. This behavior is similar to cow-shell and other LD_PRELOAD
based "CoW" solutions.

src/config.cpp

@@ -46,6 +46,7 @@ namespace mergerfs
       nullrw(false),
       ignorepponrename(false),
       security_capability(true),
+      link_cow(false),
       POLICYINIT(access),
       POLICYINIT(chmod),
       POLICYINIT(chown),