Shellchecked xdg-mime

• Replace legacy backticks with "$(
• Fix for `echo $a | sed 's/:/ /g'`; … antipattern usinf IFS=:
• Quote a lot of filepaths to avoid time of check time od use inconsistencies
• Quote all other use of variables
  • for none of them not-quoting seemend intentional
  • even where it just "doen't hurt"
  • except for for loops 😉
• disable checks for mispelled variables where the spelling is intentional
• move the local bashisms to their own lines so that when a posix shell trips over them the script shouldn't fail because of them.

Did not fix:

• local is posix "incompatible" (SC3043) (but worked around it)
• SC2086 (info): Double quote to prevent globbing and word splitting. For $xdg_redirect_output pattern.