Allow for datadirs to be colon-separated

[whot]

Useful for testing e.g. #788 with built-in tools like debug-device. The new file is in /etc/libwacom to make it system-wide accessible but debug-device only uses the git datadir by default.

This MR allows for:

./build/debug-device --database /etc/libwacom:$PWD/data /dev/input/event19

and other testing of databases distributed across multiple data dirs.