core.sh: cosmetics (indent)
core.sh: comment fix
use simple quotes if possible

In theory this is faster (no variable expansion). But I doubt we notice any performance difference.

sed -e '/[[:space:]]\+log_/{/[$'\'']/!s/"/'\''/g}'
mediafire (upload): rewrite using official API

Changes:
- added --unique switch to deal with duplicates (based on names only). Don't reupload and return original quick key.
- remote "import key" not implemented

Notes:
- Mediafire upload API: 'action_on_duplicate=skip' does not seem to work properly
- It's impossible to upload a file with 1 or 2 characters