Most system call man pages make an effort to describe potential error
return conditions for the call. getpgid(2), however, neglects to note that
the returned pid can be (-1), indicating that errno should be inspected,
in the style of getsid(2).
Probably need to copy the relevant part of getsid(2) into getpgid(2).
However, this probably involves first consulting relevant specs defining
getpgid(2) to determine if our implementation complies or not.
Bruce, can you check if this is true? If that's the case, I'll fix
the man page.