Update falco main to use falco application + cmdline_opts

Update falco's main falco_init() to use a falco::app::application and
falco::app::cmdline_opts object instead of storing all its command
line state in stack variables.

The bulk of the removed code is in usage() (not needed as cxxopt's
help() is self-documenting.) and getopt_long() which is replaced by
app.init(argc, argv).

For the most part, this is simply replacing references to local
variables (e.g. "all_events") to the bound variable inside the
cmdline_opts object (e.g. app.copts().all_events).

There are a few cases where more complex logic was used (output
formats, initializing k8s/mesos with string pointers), and those
changes are still in falco_init().

For the most part, the monolithic parts of falco_init that involve
reading config files, creating the inspector, loading rules, etc are
still present. Those will be addressed in later changes.

Signed-off-by: Mark Stemm <mark.stemm@gmail.com>