Increase portability of tmpfs implementation to support BSDs

[morgant]

Per Issue #344, this increases portability to BSDs (tmpfs implementation only, of course). Specifically, the following are addressed:

• Doesn't hard-code path to bash
• Use rm instead of non-portable unlink
• Avoid use of non-portable options:
  • find's -printf
  • cp's --reflink=auto
  • mv's --no-target-directory
  • pgrep's -c
• Only require modinfo if using OverlayFS

These changes have been extracted from my own patches for an OpenBSD port (https://github.com/morgant/my-openbsd-ports/blob/main/www/profile-sync-daemon/patches/patch-common_profile-sync-daemon_in). As noted in Issue #344, there are some additional options that differ across implementations of du & stat, but I have left those changes out as there may still be some differences between BSDs that would be best applied as OS-specific patches. Similarly, OpenBSD's rsync port doesn't support the -X option since the file system doesn't support metadata, so that should remain as an OpenBSD-specific patch.

[graysky2]

Not sure how I feel about this since I do not have any BSD boxes/VMs for testing... have you been running the modified code on BSD?

[morgant]

I have been running the modified code, but only under OpenBSD. I'd certainly prefer others running different BSDs and Linux distros confirm that this work there too.

To be honest, I wasn't sure what you'd think either and am not in a rush to have these changes merged. I mainly wanted to put the idea out there, with some suggested changes, as food for thought.

Edit: I'm happy to try to keep this PR updated as I maintain my OpenBSD port (though it hasn't been accepted into ports tree yet as there wasn't enough initial interest.)

[graysky2]

OK, let's keep this open as you said. Thank for your work to bring it to other users.