logger: optimize string initializations

Setting whole array to be completely full of nulls cannot be as quick as
making the only significant member of the array null only when needed.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
logger: set function arguments read-only when possible

Signed-off-by: Sami Kerola <kerolasa@iki.fi>
docs: mentintion default udp and tcp ports logger is using

The fact that 'syslog tcp/514' does not exist in RFS's, which has lead
to 'syslog-conn 601/tcp' be used in place could be a suprice and should
be told in manual.

Signed-off-by: Sami Kerola <kerolasa@iki.fi>