Added exists() query for remote objects and functions to simplify checking

if they still exists on the remote side. It's made without the overhead of
initiating a real call at the remote end.

Rev: lib/modules/Remote.pmod/call.pike:1.8
Rev: lib/modules/Remote.pmod/connection.pike:1.20
Rev: lib/modules/Remote.pmod/context.pike:1.13
Rev: lib/modules/Remote.pmod/obj.pike:1.6
Rev: lib/modules/Remote.pmod/remote.h:1.3