Completely rework shell scripts

The shell scripts zfs-functions et al. have multiple problems:
- Current shell code is hard to read and debug because of too much
  unoptimized code
- Some bugs (like openzfs#9168, openzfs#9183, and I guess some more but I did not
  search in the issues, yet)
- Inconsistent code style
- Use of upper case variables (very confusing at least for me, as only
  env vars or globals should be upper case (isn't this (un)written
  convention?)
- Very much missing documentation which makes it hard for new devs to
  understand what SHOULD happen (which is not always what really
  happens)
- use of evi... eval() where it is not needed at all, e.g.:
  `eval export FSTAB_dev_$i="$fs"`
- duplicated functionality: `in_mtab` vs. `is_mounted`
- inconsistent use of `mount` and `cat/read /proc/self/mounts` (which
  both return the same results. The only differene is that mount does
  some formatting (insert `on` and `type`, put `options` in brackets,
  omitting `freq` and `pass`)
- Caching of fstab and mtab in env makes everything very, very
  complicated due to special character handling and (as far as I looked
  at the code, yet) is not needed at all. (MAYBE this is really needed
  and I did not get this far, yet. We'll see.) IMHO it really does not
  matter if we call grep multiple times but can completely drop any
  `eval`. The run time difference is negligible and should be in the
  range of some 10s or 100s of milliseconds.
- ... (more to follow)

This PR does a complete rework of all existing shell scripts to fix
all those problems while keeping backwards compatibility.

Signed-off-by: Michael Niewöhner <foss@mniewoehner.de>