tool_operate: refuse (--data or --form) and --continue-at combo

libcurl assumes that a --continue-at resumption is done to continue an
upload using the read callback and neither --data nor --form use
that and thus won't do what the user wants. Whatever the user wants
with this strange combination.

Add test 426 to verify.

Reported-by: Smackd0wn on github
Fixes #11081
Closes #11083

tests/data/Makefile.inc

@@ -69,7 +69,7 @@ test390 test391 test392 test393 test394 test395 test396 test397 test398 \
 test399 test400 test401 test402 test403 test404 test405 test406 test407 \
 test408 test409 test410 test411 test412 test413 test414 test415 test416 \
 test417 test418 test419 test420 test421 test422 test423 test424 test425 \
-\
+test426 \
 test430 test431 test432 test433 test434 test435 test436 \
 \
 test440 test441 test442 test443 test444 test445 test446 \

tests/data/test426

@@ -0,0 +1,34 @@
+<testcase>
+<info>
+<keywords>
+error detection
+</keywords>
+</info>
+
+#
+# Server-side
+<reply>
+</reply>
+
+#
+# Client-side
+<client>
+<server>
+http
+</server>
+<name>
+try --data with --continue-at
+</name>
+<command>
+http://%HOSTIP:%HTTPPORT/%TESTNUMBER -d foobar -C 3
+</command>
+</client>
+
+#
+# Verify data after the test has been "shot"
+<verify>
+<errorcode>
+2
+</errorcode>
+</verify>
+</testcase>