Improve posix module

[FedericoCeratto]

Add docstrings and some helper functions.

[Araq]

I appreciate the additions but posix.nim is supposed to only be a wrapper, not provide convenience procs. For example, with your additions it is not clear anymore that none of these procs raise exceptions, because the new ones now do raise. Please move them to a new module posix_utils.nim or similar.

[juancarlospaco]

If the code is homeless and is not getting into std lib,
I maintain psutil module that has a posix module extends std libs one, more than welcome to add it there, just saying. 🙂 Awesome work whatsoever.

[Araq]

Looks good to me. Please also add it to kochdocs.nim and the changelog.

[FedericoCeratto]

@Araq updated. Should the entries in tools/kochdocs.nim sorted alphanumerically later on?

[Araq]

@Araq updated. Should the entries in tools/kochdocs.nim sorted alphanumerically later on?

Nah.