agetty: add \S to output /etc/os-release data

agetty: add \S to output /etc/os-release data

This patch allows to use a new \S or \S{VARNAME} sequence in the
/etc/issue file. The sequence prints data from /etc/os-release.  The
reason is to keep /etc/issue file distribution and release
independent.

The \S{ANSI_COLOR} is converted to the real terminal escape seq.

For example:

   \S
   Kernel \r on an \m (\l)

or more complex example:

   Welcome to \S{ANSI_COLOR}\S{NAME}\e0m \S{VERSION}
   Report bugs at \S{BUG_REPORT_URL}.

See http://www.freedesktop.org/software/systemd/man/os-release.html
fr more details about /etc/issue.

Based on patch from Bill Nottingham <notting@redhat.com>.

Signed-off-by: Karel Zak <kzak@redhat.com>