Use the Eclipse network proxy configuration for HTTP connections
Eclipse provides support for proxy configuration within the workbench,
but this information is not pushed down into the J2SE library itself
so our naive use of java.net.URL to open connections cannot get to the
proxy information.
By installing our own ProxySelector and Authenticator implementations
we can push the Eclipse proxy information down into the J2SE library
and make use of it during transport operations.
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>