Replaced libvrp with in-house implementation

libvrb's count of mmap's and munmap's was skewed towards mmap's which
resulted in virtual memory leaks.

Also dropped extra runtime dependency.
Dropped fbr_restart and fbr_cooperate

Those proved not of much use.

Also covered fbr_key_* api with basic unit test.
Fixed CMakeLists to not check for libvrb

And not to link it in as well.
throwaway
pbuilder /dev/shm workaround

As pbuilder does not mount a tmpfs onto /dev/shm, I've added an
environment variable, which is capable of overriding default fbr_buffer
file pattern. It is checked only when fbr_context is initialized.
Sketches for libeio support
Forgot to commit the tests
Extended the eio wrapper API
Fixed rebase
Fixed buffer memory benchmark
Removed libvrb from eio branch as well
Merge branch 'eio' of github.com:Lupus/libevfibers into eio

Conflicts:
	CMakeLists.txt
	bench/buffer.c
	include/evfibers/fiber.h
	include/evfibers_private/fiber.h
	src/fiber.c
	test/main.c