Remove compatibility definition of va_copy from log.c.

It's not needed here since it's being done in polipo.h.
Fix usage of va_copy.
poll(2) is defined in <poll.h>

<sys/poll.h> is deprecated, poll(2) is defined in <poll.h> in POSIX2.

This is needed for musl libc which does not provide <sys/poll.h>.