utils.c and commands.c need poll.h to compile successfully.

Signed-off-by: Jonathan Reams <jbreams@gmail.com>
Add ability for NEB modules to override host checks.

Service checks can be overriden both in
NEBTYPE_SERVICECHECK_ASYNC_PRECHECK and in
NEBTYPE_SERVICECHECK_INITIATE, but host checks only allow
ASYNC_PRECHECK. This makes hosts consistent with services.

Signed-off-by: Jonathan Reams <jbreams@gmail.com>