Yet another nail in the coffin of compel cli usage complexity.

Since commit cd1c9d9 ("compel: simplify usage wrt ids") landed,
it became obvious that the newly introduced -p option can be
made optional. First, prefix value is not very important;
second, it can be easily generated from the input (or output)
file name.

This is what this commit does, also trying to check that
the resulting prefix is adeqate for using in C code. In case
it is not (say, file names used are 1.po 1.h), an error
is printed, suggesting to use --prefix.

The commit also makes use of this functionality by removing
-p option from compel/test/infect and criu/pie Makefiles.

While at it, let's also remove -l 4 and the kludge of hiding it.
If the output generated by this is ever needed, one can easily
run "compel hgen -l4 ..." manually.

travis-ci: success for More polishing for compel cli
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Acked-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>