add zcat symlink support, suggested by @wtarreau

added some test
also updated relevant doc

+ fixed a mistake in `lz4` symlink support :
  lz4 utility doesn't remove source files by default (like zstd, but unlike gzip).
  The symlink must behave the same.

Makefile

@@ -72,14 +72,18 @@ zstdmt:
 zlibwrapper:
 	$(MAKE) -C $(ZWRAPDIR) test
 
-.PHONY: check
-check: shortest
-
-.PHONY: test shortest
-test shortest:
+.PHONY: test
+test:
 	$(MAKE) -C $(PRGDIR) allVariants MOREFLAGS+="-g -DZSTD_DEBUG=1"
 	$(MAKE) -C $(TESTDIR) $@
 
+.PHONY: shortest
+shortest:
+	$(MAKE) -C $(TESTDIR) $@
+
+.PHONY: check
+check: shortest
+
 .PHONY: examples
 examples:
 	CPPFLAGS=-I../lib LDFLAGS=-L../lib $(MAKE) -C examples/ all