Shellchecked xdg-open

• Added a .shellcheckrc file
• Replace legacy backticks with "$(
• Comment where read -r is inteletially placed and where the -r is omitted.
• Add quotes where they don't hurt
• fix iterating over user and system_dirs by using IFS properly in open_generic_xdg_mime
• Moved some local declarations out of the way
• Remove unused variable from open_envvar
• Disable checks fir the scary looking part of open_envvar
• Removed unneccessary variable in open_flatpak

Did not fix:

• local is posix "incompatible" (SC3043)