log: add option argument help

log: add option argument help

The option --log-level was not completely described in the usage text,
and it was difficult to guess the names of the log types and levels.

A new value "help" is accepted after --log-level to give more details
about the syntax and listing the log types and levels.

The array "levels" used for level name parsing is replaced with
a (modified) existing function which was used in rte_log_dump().

The new function rte_log_list_types() is exported in the API
for allowing an application to give this info to the user
if not exposing the EAL option --log-level.
The list of log types cannot include all drivers if not linked in the
application (shared object plugin case).

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>
Acked-by: Andrew Rybchenko <andrew.rybchenko@oktetlabs.ru>
Reviewed-by: David Marchand <david.marchand@redhat.com>