compel: simplify usage wrt ids

Currently, some compel internals are exposed to user API
(both C and CLI), making its usage more complicated than
it can be.

In particular, compel user have to specify a number of parameters
(names for various data) on the command line, and when in C code
assign a struc piegen_opt_t fields using the same names, without
using those identifiers anywhere else in the code.

It makes sense to hide this complexity from a user, which is what
this commit does.

First, remove the ability to specify individual names for data,
instead introducing a prefix that is prepended to all the names.
Second, generate a function %PREFIX%_setup_c_header() which does
all the needed assignments.
Third, convert users (criu/pie and compel test) to the new API.

NOTE that this patch breaks ARM, as compel hgen is not used for ARM.
This is to be fixed by a later patch in the series.

Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>