s3-rpc_server: Move pipe/handles functions

s3-rpc_server: Move pipe/handles functions

Put InternalPipes related functions in rpc_handles.c and out of rpc_ncacn_np.c
rpc_handles.c is the only file that really uses them after all and ncacn_np.c
is the wrong place for that stuff.
While ther remove unnecessary wrapper functions now that the InternalPipes
static variable is directly accessible.

Also move all pipes_struct related header stuff in its own rpc_pipes.h header.

Signed-off-by: Andreas Schneider <asn@samba.org>