We link against libuuid.so, but it was entirely internal to
libnm-core. We only exposed UUIDs in string form.

Add API to also handle UUIDs in binary form.

Note that libuuid already defines a type "uuid_t". However,
don't use it and instead use our own typedef NMUuid.
Reasons:

  - uuid.h should be internal to libnm-core (nm-utils.c specifically),
    and not be used by or exposed it other parts of the code.

  - uuid_t is a typedef for a guchar[16] array. Typedefs
    for arrays are confusing, because depending on whether
    it's an automatic variable or a pointer in a function argument,
    they behave differently regarding whether to take their address
    or not and usage of "sizeof()".