CAPS IPC library stage 1/3: The core CAPS IPC code, providing system calls

CAPS IPC library stage 1/3: The core CAPS IPC code, providing system calls
to create and connect to named rendezvous points.  The CAPS interface
implements a many-to-1 (client:server) capability and is totally self
contained.  The messaging is designed to support single and multi-threading,
synchronous or asynchronous (as of this commit: polling and synchronous only).

Message data is 100% opaque and so while the intention is to integrate it into
a userland LWKT messaging subsystem, the actual system calls do not depend
on any LWKT structures.

Since these system calls are experiemental and may contain root holes,
they must be enabled via the sysctl kern.caps_enabled.