This commit splits the '--filter' flag into two new flags: '--scope' and
'--events'.

The '--scope' flag is used to specify the scope of the events to be
captured like 'comm', 'binary', 'pid', 'uid', 'mntns', 'pidns', 'uts',
'tree', 'follow' and 'container', while the '--events' flag is used to
specify the events and their userland filters like 'args', 'retval' and
'context'.

It also removes the set option previously available for the '--filter'
flag, as it is no longer needed. The '--events' flag is now used to
specify the sets of events to be captured, e.g. '--events fs'.

Other chores:

- analyze '--event' flag now is '--events', to be consistent.
- polcy_file.go validateContext() is removed since we have similar
  validation in further stages.
- Relocates unit tests from flags_test.go to respective new correlated
  files, making it easier to maintain and extend and changes integration
  tests to use PolicyFile instead of Policy.

BREAKING CHANGE: '--filter' flag is now replaced by '--scope' and
'--events' flags and analyze '--event' flag is now '--events'.

Co-authored-by: Yaniv Agman <yanivagman@gmail.com>

Co-authored-by: Yaniv Agman <yanivagman@gmail.com>

Co-authored-by: Yaniv Agman <yanivagman@gmail.com>

Co-authored-by: Yaniv Agman <yanivagman@gmail.com>

Co-authored-by: Yaniv Agman <yanivagman@gmail.com>