pppd: fix pppol2tp option printing

pppd: fix pppol2tp option printing

PPPD crashes (SEGV) when the 'dump' or 'dryrun' options are specified and
the 'pppol2tp' option is specified.  The crash occurs because the
'pppol2tp' option value is not saved when the parameter is processed (in
the pppol2tp plugin), but is then referenced when printed.  This was
encountered using xl2tpd and the l2tp_ppp kernel module.

Modify the 'pppol2tp' plugin to save the option value.

Signed-off-by: Nathan Hintz <nlhintz@hotmail.com>
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Note: This patch resolves issue #32 (https://github.com/paulusmack/ppp/issues/32).