sysutils/rsfetch: fix memory reporting and add DragonFlyBSD support

Import upstream fix [1] to fix fix memory reporting and add DragonFlyBSD support.

[1] https://github.com/rsfetch/rsfetch/commit/2bd3ef9eaf1a0369f115022f8acb650682a1bf3e

PR: 244861
Approved by: Lewis Cook (maintainer), manu (mentor, implicit)