Fix handling of prefix type in login CLI

Key index is the common case we want to support here:

If the MSB is set, the low bits are interpreted as a 7 bit integer,
which the server should interpret as the index of the key its supposed
to use.

cli/commands.c

@@ -359,7 +359,7 @@ int login(int argc, char **argv) {
     fprintf(stderr, "handshake size is invalid in path %s\n", path);
     goto out;
   }
-  if ((handshake[0] & 0x80) != 0) {
+  if ((handshake[0] & 0x80) == 0) {
     fprintf(stderr,
             "only \"service-key-indicator\" prefix type is supported\n");
     goto out;

cli/test.sh

@@ -69,7 +69,7 @@ for n in 1 2; do
 done
 
 key="$t/vector-2/a"
-path="v2/R4cvQ1u4uJ0OOtYqouURB07hleHDnvaogAFBi-ZW48N2/myhost/exec=%2Fbin%2Fsh/"
+path="v2/x4cvQ1u4uJ0OOtYqouURB07hleHDnvaogAFBi-ZW48N2/myhost/exec=%2Fbin%2Fsh/"
 expected_tag="ZmxczN4x3g4goXu-A2AuuEEVftgS6xM-6gYj-dRrlis="
 tag=$("$binary" login --key "$key" "$path")
 if [ "$tag" != "$expected_tag" ]; then