Cleanup babeltrace-cfg, use BABELTRACE_PLUGIN_PATH

This commit changes a few things at once:

* Cleanups the help message (orders long options alphabetically,
  adds metavars where missing, etc.).

* Removes is_setuid_setgid() to use the common bt_is_setuid_setgid().

* Makes the CLI load plugins *non-recursively*, which is more in line
  with typical lists of paths such as PATH, LD_LIBRARY_PATH, PYTHONPATH,
  etc., from the following directories, in this order:

  1. BABELTRACE_PLUGIN_PATH environment variable
  2. --plugin-path option
  3. ~/.local/lib/babeltrace/plugins
  4. System path (/usr/lib/babeltrace/plugins)

  For the in-tree version (conveter/babeltrace), Makefile.am passes all
  the in-tree plugin paths so that it can find everything that is known
  within the tree.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>