Cache non-local index files always locally.
Introduce 'update' applet to force refresh of cached index files.
Fixes #19.

Tracks now probler header file dependencies and command line parameters
used to build files. E.g. changing CFLAGS rebuild all C-files. And changing
version rebuild now the files where it's used.

Fixes #24.

Currently only implement --backup to get list of (config) files in
protected directories to backup.

This also fixes a database corruption bug in database.c.

breakage and major changes.