logger: add support for --prio-prefix when logging stdin

logger: add support for --prio-prefix when logging stdin

This patch adds a new option to logger that will make it look for a
priority prefix <n> at the beginning of every line. The priority is
a single decimal number formed as explained in syslog(3).

If a prefix is found logger will log the message using the found
facility and level in that prefix, if the prefix doesn't contain a
facility the default facility specified by the -p option will be used.
If no prefix is found, logger will use the priority specified by -p.

[kzak@redhat.com: - add --prio-prefix to usage() output]

Signed-off-by: Dennis H Jensen <dennis.h.jensen@siemens.com>
Signed-off-by: Karel Zak <kzak@redhat.com>