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.

tests/.gitignore

@@ -24,6 +24,8 @@ pool
 poolTests
 invalidDictionaries
 checkTag
+zcat
+zstdcat
 
 # Tmp test directory
 zstdtest
@@ -53,6 +55,7 @@ tmp*
 *.gz
 result
 out
+*.zstd
 
 # fuzzer
 afl