Support several ABI for FreeBSD packages repository

Summary:
FreeBSD packages are sorted by major version and architecture (ABI),
for example FreeBSD:13:amd64 or FreeBSD:14:amd64.

This change allows to support several ABI for our FreeBSD repository.

Client configuration is simplified to only query one server.

The repository metadata now uses .pkg instead of .txz files,
and provide uncompressed version of meta under two names.
The Makefile is updated accordingly for the clean target.

Fixes T1937.

Test Plan:
Deployed on WindRiver

pkg update -r Nasqueron

Reviewers: DorianWinty

Reviewed By: DorianWinty

Maniphest Tasks: T1937

Differential Revision: https://devcentral.nasqueron.org/D3498