exec: Fix kill.c compile warnings by including <unistd.h>.

    Compiling kill.c
    kill.c: In function ‘__popen’:
    kill.c:129:2: warning: implicit declaration of function ‘pipe’ [-Wimplicit-function-declaration]
      if (pipe(fds) != 0) {
      ^
    ...
core: Fix pass_fd.c aliasing warnings by forcing proper packing.

    Compiling pass_fd.c
    pass_fd.c: In function ‘send_fd’:
    pass_fd.c:132:2: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
      *(int*)CMSG_DATA(cmsg)=fd;
      ^
    pass_fd.c: In function ‘receive_fd’:
    pass_fd.c:240:3: warning: dereferencing type-punned pointer will break strict-aliasing rules [-Wstrict-aliasing]
       *fd=*((int*) CMSG_DATA(cmsg));
       ^