mime: use in curl cli tool instead of form API.

Extended -F option syntax to support multipart mail messages.
-F keyword headers= added to include custom headers in parts.
Documentation upgraded.

src/tool_help.c

@@ -131,9 +131,9 @@ static const struct helptxt helptext[] = {
   {"    --false-start",
    "Enable TLS False Start"},
   {"-F, --form <name=content>",
-   "Specify HTTP multipart POST data"},
+   "Specify multipart MIME data"},
   {"    --form-string <name=string>",
-   "Specify HTTP multipart POST data"},
+   "Specify multipart MIME data"},
   {"    --ftp-account <data>",
    "Account data string"},
   {"    --ftp-alternative-to-user <command>",