Shellchecked xdg-utils-common.in

With an always quote, unless intended policy except for SC2268 (x prefixes)

Main changes:

• Replace old "` with new "$( syntax
• Replace comparisons with empty strings with [ -n and [ -z as appropriate
• Replace $@ with $* fir the message in the exit_* functions as merging arguments is intended there
• Quote one occurrence of $DESKTOP in detectDE
• Introduce quotes in kfmclient_fix_exit_code
• Added spellcheck directives where checks shouldn't apply
  • Intentionally no backslash ignoring in first_word
  • exit_success is okay to be called without arguments
  • xdg_redirect_output appears unused, but isn't
  • $DE is not misspelled