Mark `clear` as `deprecated`
Added deprecated message and change alias syntax
Updated comment to reflect 6 month deprecation policy
Merge pull request #757 from JinShil/master

Mark `clear` as `deprecated`
Add some missing OS X declarations from <sys/signal.h> and <unistd.h>
Merge pull request #759 from CyberShadow/osx-signal-unistd

Add some missing OS X declarations from <sys/signal.h> and <unistd.h>