Fixes/shellcheck

Shellcheck fixes

This PR has lots of fixes based on running shellcheck on Pkg:

shellcheck --color=always --exclude=SC2086,SC1090,SC1091,SC2155,SC2002,SC2162,SC2006 /usr/sbin/pkg | less

Add and remove the SC stuff as needed when fixing different things...

Fixes in this PR:

• fixes in getting package information:
  • package installed or not
  • package repo
  • package contents
  • package filename (new function get_pkg_filename, respects repo fallback order)
  • package versions
• slightly better error checking
  • should be a little bit more reliable
• faster search commands: pkg s <term>, pkg sa <term>, pkg ss <term>, pkg ssa <term>
• faster installation and uninstallation of packages
• pkg what-needs <pkgname> now much better (faster and more accurate)
  • therefore pkg rm <pkgname will be much faster
• fixed output for pkg PS <pkgname> (nicely align all deps, not only first 10 lines)
• fixes in setting up and running petbuild

Other changes

• fixed various code errors/typos
• fewer sub-shells, a little bit faster
• removed some old code
• simplified some code
• cleaned up long lines, made them < 80 chars

TODO

• fix pkg_update, still not reporting final versions quite right
  • it's kind of respecting fall back order, and also not ..

In a future PR

Go through again and reduce number of sub-shells, but keep the code simple - no exotic, unreadable stuff.