fix: fix available memory output for NetBSD and use whoami for Android

[rilysh]

• NetBSD doesn't expose a sysctl entry for available memory and the kernel module isn't callable from shell script. Use vmstat(1) with awk(1) to fix the current issue.

• Replace "USER" variable entry with whoami(1). It seems some shells don't interpret ${$(...)}, and we can replace this by using whoami(1). It's available in coreutils, and most other Unix-like userlands.

• Only redirect stdout to /dev/null as command -v doesn't give a stderr and only a return value is provided by it. This fixes printing path output of getprop when running NerdFetch on Android.

[ThatOneCalculator]

Merged manually due to conflicts. Thank you!