refactor the install script

[h3o66]

I refactored the install script a bit with the following points:

• SC2148 add shebang header
• SC2166 redo if conditions to have them well defined
• SC2006 change notation to remove legacy backticks
• SC2002 remove useless usage of cat
• SC2143 only use grep -q instead of comparing output with [ -n .. ]

These are all Shellcheck point what where corrected.

Note: i did not check if this works for all supported distros, but i do not expect here any major issues