Allow rdefs in subdirectories in makefile-engine.

Also use finddir instead of hardcoded paths for drivers and locale
catalog helper rules.

Fixes #11115. Thanks to Waddlesplash and Puckipedia for investigating
the issue.
SerialConnect: use ints for font metrics

As pointed by Ingo, using a float here is useless because we want each
line to be the same integer number of pixels. Now thigs are drawn
properly and the view has the exact same size as Terminal's one (tested
with font sizes 10 and 18, so different leading values don't seem to be
a problem).

Thanks for reviewing!
Build fix.