coap-client: The -U option doesn't expect an argument
coap-client: The -U option doesn't expect an argument

Thus it must not be postfixed with a colon character in the optstring.
While at it move the option to the beginning of the opstring to the
other options which don't expect an argument.

Without this commit the -U option is unusable.

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap