a follow on commit to kern_sig.c
- signotify() now operates on a thread since unmasked pending signals are
stored in the thread.
- PS_NEEDSIGCHK moves to TDF_NEEDSIGCHK.
prototyped functions to get a sigset_t, and further to check for any
queued signals, rather than an empty signal set, to go with the move
to signal queues rather than signal sets.
Use mchain API to work with mbuf chains.
Do not depend on INET and IPX options.
Allocate ncp_rq structure dynamically to prevent possible stack overflows.
Let ncp_request() dispose control structure if request failed.
Move all NCP wrappers to ncp_ncp.c file and all NCP request processing
functions to ncp_rq.c file.
Improve reconnection logic.
Misc style fixes.
