signal: split user and system signals in two distinct intervals

signal: split user and system signals in two distinct intervals

Reorganize the bits of sigmask_t so that signals that can be used by
drivers or user applications and signals reserved for internal system
use are placed in two distinct intervals:

  * * * . . . . .
 |_|_|_|_|_|_|_|_|
  7 6 5 4 3 2 1 0  sigmask_t bits

 (*) bits [5-7] are reserved for internal system usage (SIG_TIMEOUT, etc.)
 (.) bits [0-4] are available for driver and user applications usage

This makes easier to allocate a range of contiguous signals. This can be
used for example to wait the occurrence of multiple events, multiplexing
them with multiple signal bits.

git-svn-id: https://src.develer.com/svnoss/bertos/trunk@4756 38d2e660-2303-0410-9eaa-f027e97ec537