Fixed agent.debug option in internal_options.conf

Doesn't look like agent.debug in the internal_options.conf ever worked.
This patch changes the behavior of the NIX agent to use whatever debeug
level is specified on the command line first. If no debug level was set
it will then look at the debug level set in the internal_options.conf.
Declare debug_level to 0 in ossec-agentd for NIX
Move debug determination logic so it happens before first debug
statement