Add support for configuration files

Implementation changes for usage of simple configuration files. Before
parsing the command line options, either default configuration files
(/etc/criu/default.conf, $HOME/.criu/default.conf; in this order) are
parsed, or a specific config file passed by the user. Two new options are
introduced: "--config FILEPATH" option allows users to specify a single
configuration file they want to use; and "--no-default-config" option to
forbid the parsing of default configuration files. Both options are to be
passed only via the command line.

Usage of configuration files is not mandatory to keep backwards
compatibility. The implementation of this feature tries to be compatible
with command line usage -- the user should get the same results whether
he passes the options (in the right order of parsing) on command line or
writes them in config files. This allows the user to:

1) Override boolean options if needed
2) Specify partial configuration for options that are possible to pass
   several times (e.g. "--external"), and pass the rest of the options
   based on process runtime by command line

Configuration file syntax allows comments marked with '#' sign, the rest
of the line after '#' is ignored. The user can use one option per line
(with argument supplied on the same line if needed, divided with whitespace
characters), the options are the same as long options (without the "--"
prefix used on command line).

Configuration file example (syntax purposes only, doesn't make sense):

$ cat ~/.criu/default.conf
tcp-established
work-dir /home/<USERNAME>/criu/work_directory
extra # inline comment
no-restore-sibling
tree	111111

Signed-off-by: Veronika Kabatova <vkabatov@redhat.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>