Bug#683908: gpsprune: doesn't properly use $http_proxy

[debian at activityworkshop.net]

Thank you for that.  That looks good, simpler than using an extra grep. 
I've not seen those sed options before.

One minor suggestion, it might be worth changing:
  proxyport=$(echo $http_proxy | sed -n 
's?http://[^:]\+:\([0-9]\+\)?\1?p')
to
  proxyport=$(echo $http_proxy | sed -n 
's?http://[^:]\+:\([0-9]\+\).*?\1?p')

It might not be necessary, but just in case the http_proxy is 
incorrectly set with an extra / or something after the port number, then 
the extra ".*" will catch and remove it.