syslog: do not print bufp and len for commands that ignore them

syslog: do not print bufp and len for commands that ignore them

* syslog.c (SYS_FUNC(syslog)): Defer printing of comma after the first
argument to the specific command handlers, return RVAL_DECODED without
additional printing for SYSLOG_ACTION_CLOSE, SYSLOG_ACTION_OPEN,
SYSLOG_ACTION_CLEAR, SYSLOG_ACTION_CONSOLE_OFF,
SYSLOG_ACTION_CONSOLE_ON, SYSLOG_ACTION_SIZE_UNREAD,
SYSLOG_ACTION_SIZE_BUFFER.
* tests/syslog.c: Add checks.