src/lib: Convert libc::rlim_t to u64

[mxinden]

Previously a27a68d wrongly assumed libc::rlim_t to be a u64 on all
platforms. Instead the following is the case:

• x86_64-unknown-linux-gnu: u64
• x86_64-apple-darwin: u64
• i686-unknown-linux-musl: u64
• i686-unknown-linux-gnu: u32
• ...

There are different options in order to compile on all platforms:

• Return usize. While usize would work for most platforms,
  i686-unknown-linux-musl is an exception. Thus casting would be needed
  which introduces yet another way to panic. In addition usize should be
  used to index into collections, not so much to represent actual data.

• Return libc::rlim_t. This type is not present on all platforms. In
  addition exposing a libc type in the public interface unnecessarily
  ties us to libc.

• Return u32. Would require a fallible cast on all u64 platforms.

• Return u64. Supported on all platforms. It is not the most space
  efficient option.

Given that memory efficiency (u32 vs u64) is irrelevant in this
context, compared to the involved system calls, this commit converts the
limit to u64 before returning for all platforms.

Fixes #5.

[mxinden]

Tested on the following platforms. Let me know in case I am missing one.

➜  fdlimit git:(convert-to-u64) cargo build --target x86_64-unknown-linux-gnu

   Compiling libc v0.2.76
   Compiling fdlimit v0.2.0 (/home/mxinden/code/github.com/paritytech/fdlimit)
    Finished dev [unoptimized + debuginfo] target(s) in 0.73s
➜  fdlimit git:(convert-to-u64) cargo build --target x86_64-apple-darwin

   Compiling fdlimit v0.2.0 (/home/mxinden/code/github.com/paritytech/fdlimit)
    Finished dev [unoptimized + debuginfo] target(s) in 0.43s
➜  fdlimit git:(convert-to-u64) cargo build --target i686-unknown-linux-musl

   Compiling fdlimit v0.2.0 (/home/mxinden/code/github.com/paritytech/fdlimit)
    Finished dev [unoptimized + debuginfo] target(s) in 0.12s
➜  fdlimit git:(convert-to-u64) cargo build --target i686-unknown-linux-gnu

   Compiling fdlimit v0.2.0 (/home/mxinden/code/github.com/paritytech/fdlimit)
    Finished dev [unoptimized + debuginfo] target(s) in 0.08s
➜  fdlimit git:(convert-to-u64) cargo build --target armv7-unknown-linux-gnueabihf

    Finished dev [unoptimized + debuginfo] target(s) in 0.01s

[decathorpe]

Oh, it's really weird that it's not even consistent across 32-bit targets ...
So, always converting to the larger data type (u64) sounds good to me. 👍

[mxinden]

@NikVolf could you give this a review?