Core: support several "error_log" directives.

Core: support several "error_log" directives.

When several "error_log" directives are specified in the same configuration
block, logs are written to all files with a matching log level.

All logs are stored in the singly-linked list that is sorted by log level in
the descending order.

Specific debug levels (NGX_LOG_DEBUG_HTTP,EVENT, etc.) are not supported
if several "error_log" directives are specified.  In this case all logs
will use debug level that has largest absolute value.