Implement chcpu

[alxndrv]

This PR implements pretty much the entirety of chcpu.

Some things still missing:

• Tests: I'm not entirely sure how to properly test something which is so tightly coupled to system internals.
• Exit codes: chcpu should return a non-zero exit code in some edge cases. This implementation does check for most such cases, but pretty much always returns a success exit-code. I'm guessing that uucore::error::UResult can be used to solve this somehow without having to manually call std::process::exit().

[sylvestre]

in general, to test such things, you either try to mock up
and you do your best to look at the error management

[alxndrv]

@sylvestre Thanks for the review, I'll implement your suggestions a bit later. Since we're on the topic of deduping: there's some code here that's copy-pasted from my work on lscpu (the parse_cpu_list() function). Should there be some kind of "lib" crate for common stuff like this? These kinds of CPU lists come up in a bunch of Linux utils and theres no sense in implementing it all over again every time. There's other similar examples: for example the mount-related utils all have to parse mount entries from /proc/<pid>/mountinfo and could benefit from having a shared lib.