Updater: check whether system() is useable.

system() was disabled on my system for security reasons, but the
updater would just continue, only to find out that the tarball
hasn't been extracted.
Merge pull request #410 from bram85/master

Updater: check whether system() is useable.
edit feed: smaller buttons for icon upload; allow uploading larger favicons
include subcategories in generated category feeds