add askpass named pipe support (GSUDO_ASKPASS_NAMED_PIPE)

[awakecoding]

We're integrating gsudo in Remote Desktop Manager, and one thing we're missing is a way to inject the password to launch the process as another user. One way we've done this in the past was to patch software to accept a named pipe name through an optional environment variable, and treat it as an "askpass" named pipe that just serves the password once. The named pipe is only used as a replacement for the current prompt if the GSUDO_ASKPASS_NAMED_PIPE environment variable has been set.

Alternatively, we could add a command-line parameter to pass the optional "askpass named pipe" name, but we're happy with just this environment variable, unless you wish to have it changed or done differently.