maint: refactor tail.c to use iopoll

maint: refactor tail.c to use iopoll

* src/tail.c (check_output_alive): Reuse iopoll()
rather than directly calling poll() or select().
* src/iopoll.c (iopoll): Refactor to support non blocking operation,
or ignoring descriptors by passing a negative value.
* src/iopoll.h (iopoll): Adjust to support a BLOCK parameter.
* src/tee.c (tee_files): Adjust iopoll() call to explicitly block.
* src/local.mk: Have tail depend on iopoll.c.