Simplify linked list of I/O watchers and its garbage collect mechanism.

Simplifed uev_io_t by removing uev_io_rem_t completely.  A refactor that
would not have been possible without the BSD queue.h API.  This makes
the code readable and thus maintainable and less error prone.

Signed-off-by: Joachim Nilsson <troglobit@gmail.com>