security/i2pd: make the port compatible with syslogging

i2pd supports logging via syslog. This may be enabled
with its configuration file, but port's startup script forces
logging to a file using command line switches that override
configuration file.

This change adds syslogging support with following rc.conf setting:

i2pd_logfile="syslog" # translates to: --log syslog

or

i2pd_logfile="syslog:warn" # translates to: --log syslog --loglevel warn

The change is backward compatible with previous settings.

PR: 252159
Approved by: amdmi3 (maintainer timeout over 3 weeks)