fixes for blacklist and merge

Improved Blacklisting:

• fixed: respect black list in pkg_uninstall() and pkg_update()
• new: new blacklist file at ~/.pkg/blacklisted_packages
  • must contain package names (name only, no versions, eg 'gimp'), one on each line
• new: created a PKG_BLACKLIST env var:
  • it's value is the combined contents of ~/.pkg/blacklisted_packages and $PKG_NAME_IGNORE
• new: added commands pkg blacklist <pkgname> and pkg whitelist <pkgname>
• updated: faster to check if package is in blaclist
• updated help and usage info

Fixes:

• small clean up in get_pkg()
• check if DISTRO_TARGETARCH is ARM, set DBINARCH, etc (needs more work)
• strip arch from filenames (fixes getting proper pkg names)
  • lots of funcs will benefit from these fixes:
    • will improve matching of relevant packages
    • will improve filtering, sorting, removing duplicates
• fix pkg merge (fix getting list of deps, no duplicates)