The space calculation must use the same length as the eventual
return value.

Assumed initial character was a separator when writing the option,
and skipped over it.

Replaces all occurences of COAP_CT_.* with COAP_FORMAT_.*

For CoAP, there is actually a difference between
`/some/path` and `/some/path/`. This needs to be reflected
when parsing the URI and location path options from a given
string.

- add generic string put and get functions
- add location path and location query options
- add dedicated functions for getting and setting
  URI query, URI path, location query, and location path
  options

Includes string and uint options.

The nanocoap_get function is refactored to split of the request part
into a separate function for reuse by other modules. Support for
retransmissions when the received frame is malformed is dropped as it
was broken anyway.