compel: Reshuffle the directories structure

Here we rather suffle source code into directories
preparing ground for future work.

Basically all this files movements should end up in the
following compel/ tree structure

compel/
├── arch
│   ├── aarch64
│   │   ├── plugins
│   │   │   └── std
│   │   └── src
│   │       └── lib
│   ├── arm
	...
│   ├── ppc64
	...
│   └── x86
	...

	This is architectural part, where each arch consists of
	plugins/, and src/. src/ stands for code needed by compel
	cli + lib

├── include
│   ├── compiler.h -> ../../criu/include/compiler.h
│   ├── elf32-types.h
│   ├── elf64-types.h
│   ├── int.h -> ../../criu/include/asm-generic/int.h
│   ├── piegen.h
│   ├── shmem.h
│   └── uapi
│       ├── compel.h
│       └── plugins.h

	Common includes + uapi

├── plugins
│   ├── fds
│   ├── shmem
│   └── std

	Plugins source code

└── src
    ├── lib
    │   ├── handle-elf-32.c -> handle-elf.c
    │   ├── handle-elf-32-host.c -> handle-elf-32.c
    │   ├── handle-elf.c
    │   └── handle-elf-host.c -> handle-elf.c

	compel library

    ├── main.c
    ├── main-host.c -> main.c

	compel cli

    └── shared
        └── fds.c

	shared code between plugins and compel cli

Note: cross-compile won't work for a while.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>