Syslog logging support

[alex-krash]

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In this PR a logging to syslog has been implemented. For backward compatibility, it is turned on with currently hidden option use_syslog.

[alexey-milovidov]

Why we have

options SYSLOG_CONS | SYSLOG_PID
and LOG_DAEMON facility

for local syslog, but

LOG_USER facility

for rsyslog?

[alex-krash]

1. rsyslog does'not support SYSLOG_CONS|SYSLOG_PID options (for local, these option's combination has been taken from existing one implementation)
2. According to facility, I've picked up existing for local syslog, and used Poco's default for rsyslog. Possibly, to simplify and unify usage, we may use LOG_DAEMON for rsyslog as well.

[alexey-milovidov]

So, everything is perfect.