This patch updates Makefile for measuring code coverage in CRIU.

criu binary should be compiled with option --coverage and linked with -lgcov
option. lcov utility creates HTML pages containing the source code annotated
with coverage information.

make GCOV=1
make test
make gcov
Look at gcov/html/index.html

Changes:
- disable optimization on compilation for gathering coverage data
- mention an option for coverage in help
- cleanup the files produced for code coverage
- make path to coverage files produced by tests in ns independent on environment
- add an option lcov_branch_coverage as branch coverage disabled by default

Signed-off-by: Sergey Bronnikov <sergeyb@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>