Re-integrate FEAT_FILTERPIPE code

This feature was accidentally removed when doing the initial import from vim. It
makes vim use pipes instead of temporary files for filtering buffers through
shell commands.

I found that this was missing when looking for references of
SHELL_READ/SHELL_WRITE outside mch_call_shell`.

When `mch_call_shell` is reimplemented on top of libuv process management
facilities, pipes will always be used for communication with child processes so
it makes sense to enable the feature permanently.

src/eval.c

@@ -10252,6 +10252,7 @@ static void f_has(typval_T *argvars, typval_T *rettv)
     "extra_search",
     "farsi",
     "file_in_path",
+    "filterpipe",
     "find_in_path",
     "float",
     "folding",