plugins/howvec: adapting to the new argument passing scheme

Correctly parsing plugin argument since they now must be provided as
full-form boolean parameters, e.g.:
    -plugin ./contrib/plugins/libhowvec.so,verbose=on,inline=on

Also, introduced the argument "count" that accepts one opt to count
individually at a time.

Signed-off-by: Mahmoud Mandour <ma.mandourr@gmail.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Tested-by: Alex Bennée <alex.bennee@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20210730135817.17816-8-ma.mandourr@gmail.com>