efivar: Add --quiet to not report errors

Add -q/--quiet flag to the command line. With it, errors are not reported at
all. Instead nothing is printed and the exit code is non-zero.

Reviewed by: markj
Sponsored by: Netflix, Inc
Differential Revision: https://reviews.freebsd.org/D29619

(cherry picked from commit 3c0dcbfc8501155af2c5461de10fd01c615524a0)