Simplified server pipe implementation by changing arguments passed down

Simplified server pipe implementation by changing arguments passed down
through to the individual pipe api calls.  Instead of passing two
prs_struct pointers, we now pass the pipes_struct pointer which contains
the former information as well as other useful stuff like the vuid.

Removed dependency on extern current_user and fetch the vuid from the
pipes_struct.
(This used to be commit 1b06451fff11f54be7def4a427a1528bbb52f3d7)