radicle: use rlim_t when working with limits — heartwood

rlimit values have different types on Linux and FreeBSD, as well as 64-bit and 32-bit platforms, so stick to the type rlim_t provided by the libc crate.

 use std::fmt;
 use std::io;
 use libc::{getrlimit, rlimit, setrlimit, RLIMIT_NOFILE};
 use libc::{getrlimit, rlim_t, rlimit, setrlimit, RLIMIT_NOFILE};
 #[cfg(not(target_os = "freebsd"))]
 type Int = u64;
 #[cfg(target_os = "freebsd")]
 type Int = i64;
 type Int = rlim_t;
 /// Sets the open file limit to the given value, or the maximum allowed value.
 pub fn set_file_limit<N>(n: N) -> io::Result<Int>