curl: two new command line options for etags

--etag-compare and --etag-save

Suggested-by: Paul Hoffman
Fixes #4277
Closes #4543

src/tool_help.c

@@ -131,6 +131,10 @@ static const struct helptxt helptext[] = {
    "EGD socket path for random data"},
   {"    --engine <name>",
    "Crypto engine to use"},
+  {"    --etag-save <file>",
+   "Get an ETag from response header and save it to a FILE"},
+  {"    --etag-compare <file>",
+   "Get an ETag from a file and send a conditional request"},
   {"    --expect100-timeout <seconds>",
    "How long to wait for 100-continue"},
   {"-f, --fail",