In shared multi-processing mode, cancelling remote threads or updating
their scheduling parameters require going through an agent thread,
running on the process they belong to.

This patch series introduces such mechanism, so that
threadobj_cancel() and threadobj_set_priority() work transparently,
regardless of whether or not the target thread runs in the current
process.

CAUTION: remote operations are implemented by a fully asynchronous
protocol. The status returned to a caller for an operation affecting a
remote thread, tells whether the request was successfully sent to the
agent, but does not reflect the final operation status in the remote
process. The remote agent will issue a warning message in case of
failure to carry out an operation though.