io::Process needs to expose more functionality

[alexcrichton]

Some missing bits I just discovered:

• UV_PROCESS_SETUID - someone's gonna want this
• UV_PROCESS_SETGID - someone's gonna want this
• UV_PROCESS_DETACHED - how else do you fork off a daemon?

Note that this doesn't just apply to libuv, but also to libnative. The unix libnative versions are probably straightforward (just take a look at the libuv implementation).

Marking as E-mentor (I'll mentor)