build-debian.sh: Save .deb files; sign output conditionally

Pass "-uc -us" to debuild if gpg is not set up.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
release.sh: Sign output conditionally

Add the same check that's in build-debian.sh

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
Fix up return value checks on read() / fread()

fread() might return 0 on error, or it might set ferror().  It will never
return a negative number.

read() returns a ssize_t.

Signed-off-by: Kevin Cernekee <cernekee@gmail.com>
travis: Initial commit