dev: fix shellcheck lint issues

[alexandear]

This PR fixes shellcheck linter issues:

• Quote this to prevent word splitting
• In POSIX sh, == in place of = is undefined
• Double quote to prevent globbing and word splitting

The function log_prefix() is removed because it's redefined below.

Details

❯ shellcheck install.sh

In install.sh line 154:
  echo "$0"
  ^-------^ SC2317 (info): Command appears to be unreachable. Check usage (or ignore if invoked indirectly).


In install.sh line 203:
    sunos) [ $(uname -o) == "illumos" ] && os=illumos ;;
             ^---------^ SC2046 (warning): Quote this to prevent word splitting.
                         ^-- SC3014 (warning): In POSIX sh, == in place of = is undefined.


In install.sh line 221:
  echo ${arch}
       ^-----^ SC2086 (info): Double quote to prevent globbing and word splitting.

Did you mean: 
  echo "${arch}"

For more information:
  https://www.shellcheck.net/wiki/SC2046 -- Quote this to prevent word splitt...
  https://www.shellcheck.net/wiki/SC3014 -- In POSIX sh, == in place of = is ...
  https://www.shellcheck.net/wiki/SC2086 -- Double quote to prevent globbing ...