client.c: fix bug in Accept option parser
client.c: fix bug in Accept option parser

cmdline_content_type() treated option numbers as 8-bit integers and
therefore could not parse values >255. Now, option numbers are
represented as uint16_t to allow for numbers between 0 and 65535.

Moreover, as RFC 7252 has made the Accept option non-repeatable,
cmdline_content_type() now will only add a single Accept option.

authmillenon/libcoap,authmillenon/libcoap,authmillenon/libcoap