Use 'frame' instead of 'signal', take out stdlib.h in silk/.

On MacOS, stdlib.h ends up including sys/signal.h, generating
warnings about the local variables called 'signal' shadowing
the global symbol signal(3).

This was originally done in 86476906 but it missed some use
of 'signal' in prototypes in headers where it didn't cause
warnings. Later the prototypes were moved around and the
warnings came back.

This also cleans up some cases in where stdlib.h was used
but shouldn't be required.