Get hostname in $HOST or through hostname

Summary:
This covers probably more case:

• on Linux we've already $HOSTNAME
• on FreeBSD we can read $HOST
• on macOS we can call hostname

There is no need to query systemd flavour of hostname, as Linux systems
already publish the environment variable.

Test Plan: Tested on macOS

Reviewers: dereckson

Reviewed By: dereckson

Differential Revision: https://devcentral.nasqueron.org/D3162