kevent: Generic event handling mechanism.

Generic event handling mechanism.

Changes from 'take7' patchset:
* new mmap interface (not tested, waiting for other changes to be acked)
- use nopage() method to dynamically substitue pages
- allocate new page for events only when new added kevent requres it
- do not use ugly index dereferencing, use structure instead
- reduced amount of data in the ring (id and flags), maximum 12 pages on x86 per kevent fd

Changes from 'take6' patchset:
* a lot of comments!
* do not use list poisoning for detection of the fact, that entry is in the list
* return number of ready kevents even if copy*user() fails
* strict check for number of kevents in syscall
* use ARRAY_SIZE for array size calculation
* changed superblock magic number
* use SLAB_PANIC instead of direct panic() call
* changed -E* return values
* a lot of small cleanups and indent fixes

Changes from 'take5' patchset:
* removed compilation warnings about unused wariables when lockdep is not turned on
* do not use internal socket structures, use appropriate (exported) wrappers instead
* removed default 1 second timeout
* removed AIO stuff from patchset

Changes from 'take4' patchset:
* use miscdevice instead of chardevice
* comments fixes

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.