[FEAT] add pkgsrc support

[nbvins]

Description

Fastfetch currently uses src/detection/packages_obsd.c to detect the package count on NetBSD; this works, given the common ancestry and similarities of packaging tools between OpenBSD ports and pkgsrc.

The only downside is that the packaging system reported is always 'pkg' regardless of the target system. For consistency, fastfetch should report 'pkgsrc' instead on NetBSD.

PS: Besides NetBSD, pkgsrc also provide binaries for macOS, Linux (CentOS/RHEL) and Illumos. This leaves room for discussion with regard to the possibility of building support for pkgsrc on such systems.

Motivation

For reasons of consistency. I reckon this is more of a cosmetic change than else, so I've flagged it as a feature request.