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 'take3' patchset:
 * removed serializing mutex from kevent_user_wait()
 * moved storage list processing to RCU
 * removed lockdep screaming - all storage locks are initialized in the same function, so it was learned
	to differentiate between various cases
 * remove kevent from storage if is marked as broken after callback
 * fixed a typo in mmaped buffer implementation which would end up in wrong index calcualtion

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
 * added optimisation, which is aimed to help when a lot of kevents are being copied from userspace
 * 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.

Signed-off-by: Evgeniy Polyakov <redacted>