Bug#617285: pptpsetup: use absolute path for pptp in tunnel file

[James Cameron]

Thanks, taken upstream.

63eea745af41f781b60cfdd39813e4f000ead5f0

pptpsetup - use absolute path to pptp

/usr/sbin may not be in PATH, especially when pptp is configured for use
by non-root user.

See discussion in Debian bug 617285.