std.os.linux.setsid(): return raw syscall0 result

When not linking libc on 64-bit Linux and calling posix.setsid(),
we get a type error at compile time inside of posix.errno().  This
is because posix.errno()'s non-libc branch expects a usize-sized
value, which is what all the error-returning os.linux syscalls
return, and linux.setsid() instead returned a pid_t, which is only
32 bits wide.

This and the other 3 pid-related calls just below it (getpid(),
getppid(), and gettid()) are the only Linux syscall examples here
that are casting their return values to pid_t. For the other 3
this makes sense: those calls are documented to have no possible
errors and always return a valid pid_t value.

However, setsid() actually can return the error EPERM, and
therefore needs to return the raw value from syscall0 for
posix.errno() to process like normal.

Additionally, posix.setsid() needs an @intCast(rc) for the success
case as a result, like most other such cases.

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