hs20-osu-client: Validate HTTPS server certificate by default (browser)

hs20-osu-client: Validate HTTPS server certificate by default (browser)

This changes "hs20-osu-client browser <URL>" behavior to validate the
HTTPS server certificate against the system trust roots. The new command
line argument -T can be used to disable this validation.

This does not change behavior for SPP/OMA-DM triggered OSU operation,
i.e., they continue to not mandate server certificate validation for now
to avoid breaking existing test cases.

Signed-off-by: Jouni Malinen <j@w1.fi>