kevent: Generic event handling mechanism.

Generic event handling mechanism.

I send this patchset for comments and review, it still contains AIO and aio_sendfile() implementation on top of get_block() abstraction, which was decided to postpone for a while (it is simpler right now to generate patchset as a whole, when kevent will be ready for merge, I will generate patchset without AIO stuff). It does not contain mapped buffer implementation, since it's design is not 100% completed, I will present that implementation in the third patchset.

Changes from 'take2' patchset:
* split kevent_finish_user() to locked and unlocked variants
* do not use KEVENT_STAT ifdefs, use inline functions instead
* use array of callbacks of each type instead of each kevent callback initialization
* changed name of ukevent guarding lock
* use only one kevent lock in kevent_user for all hash buckets instead of per-bucket locks
* do not use kevent_user_ctl structure instead provide needed arguments as syscall parameters
* various indent cleanups
* mapped buffer (initial) implementation (no userspace yet)

Changes from 'take1' patchset:
- rebased against 2.6.18-git tree
- removed ioctl controlling
- added new syscall kevent_get_events(int fd, unsigned int min_nr, unsigned int max_nr, unsigned int timeout, void __user *buf, unsigned flags)
- use old syscall kevent_ctl for creation/removing, modification and initial kevent initialization
- use mutuxes instead of semaphores
- added file descriptor check and return error if provided descriptor does not match kevent file operations
- various indent fixes
- removed aio_sendfile() declarations.

Thank you.