pppd: allow use of arbitrary interface names

pppd: allow use of arbitrary interface names

This is a modified version of a patch from openSUSE that enables PPP interfaces
to be called arbitrary names, rather than simply pppX where X is the unit
number.

The modifications from the stock openSUSE patch are:
- refresh patch on top of 018_ip-up_option.diff
- fix a printf format-string vulnerability in pppd/main.c:set_ifunit()
- clarify the pppd.8 manpage additions
- patch pppstats/pppstats.c to query renamed interfaces without complaint

Origin: SUSE
Bug-Debian: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=458646
Forwarded: no
Reviewed-by: Chris Boot <bootc@debian.org>
Signed-off-by: Paul Mackerras <paulus@ozlabs.org>