PCNTL : Improve sigchld handling

PCNTL extension is missing WCONTINUED constant and WIFCONTINUED macro (available since kernel 2.6.10) to provide a better control over "forked" processes using the php cli binary.

This pull request propose to export a new function to php userland : pcntl_wifcontinued and a new constant  : WIFCONTINUED.

This would allow a better control of SIGCHLD signal handling whenever a process gets stopped and then resumed. At this time, a PHP program (process) cannot take action and read the child process status when a child is resumed by a SIGCONT.