process: i32 vs u32

[troelsarvin]

procfs::process::Task's pid is i32.

But procfs::process::Task::children() returns a result with a vector of u32.

How can there be a difference? Shouldn't it be the same type, and shouldn't it be a signed value? According to POSIX, glibc, as well as FreeBSD's libc, pid_t is a signed integer.