src: fix process.getuid() return value

And process.getgid() too.

Commit ed80638 changed fs.chown() and fs.fchown() to only accept
unsigned integers. Make process.getuid() and process.getgid() follow
suit.

This commit should unbreak npm on OS X - it's hitting the new 'uid must
be an unsigned int' check when installing as e.g. user 'nobody' (which
has an UID of -2 in /etc/passwd or 4294967294 when cast to an uid_t.)

Fixes #5904.