posix: headers: create a more conformant posix_signal.h

Create a header for satisfying POSIX conformance requirements of signal.h
that is named posix_signal.h rather than signal.h.

The primary reason for doing so, is that the de-facto owner of signal.h is
the C library.

This new header only defines required POSIX symbols that form a strict
superset over the ISO C signal.h symbols.

Signed-off-by: Chris Friedt <[email protected]>

Author: cfriedt