Make --enable-debug fail when given bogus args

Make --enable-debug fail when given bogus args

Currently, bogus options to --enable-debug become --disable-debug. That
means that passing --enable-debug=true is analogous to --disable-debug,
but the result is counterintuitive. We switch to AS_CASE to allow us to
fail when given a bogus option.

Also, we modify the text printed to clarify that --enable-debug enables
assertions.

Reviewed-by: Chunwei Chen <tuxoko@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Yao <richard.yao@clusterhq.com>
Closes #2734