copy: change the various copy_xyz() calls to take a unified flags parameter

This adds a unified "copy_flags" parameter to all copy_xyz() function
calls, replacing the various boolean flags so far used. This should make
many invocations more readable as it is clear what behaviour is
precisely requested. This also prepares ground for adding support for
more modes later on.

src/sysusers/sysusers.c

@@ -211,7 +211,7 @@ static int make_backup(const char *target, const char *x) {
         if (r < 0)
                 return r;
 
-        r = copy_bytes(src, fileno(dst), (uint64_t) -1, true);
+        r = copy_bytes(src, fileno(dst), (uint64_t) -1, COPY_REFLINK);
         if (r < 0)
                 goto fail;