Call umount directly when needed
Call umount directly when needed

Fixes https://github.com/fhunleth/fwup/issues/55 and removes a call to
system() that was never good in the first place.

fhunleth/fwup,michaelkschmidt/fwup,fhunleth/fwup,fhunleth/fwup,michaelkschmidt/fwup,michaelkschmidt/fwup