more careful gpst_esp_catch_probe()

more careful gpst_esp_catch_probe()

Previous version of gpst_esp_catch_probe would catch/filter *any* ping reply sent over the tunnel
from the "magical" ESP "gateway" address. (Heavy-handed scare quotes intentional.)

This may result in confusing behavior in some testing/debugging scenarios, as described in this thread:

    http://lists.infradead.org/pipermail/openconnect-devel/2019-April/005294.html

This patch modifies gpst_esp_catch_probe() to only catch ping replies if they also contain the
appropriate magic payload.

Signed-off-by: Daniel Lenski <dlenski@gmail.com>