FTBFS on GNU/Hurd: no PATH_MAX

PATH_MAX is undeclared on GNU/Hurd, thus tor fails to compile since version 0.3.3

A build log of debian is available at [https://buildd.debian.org/status/fetch.php?pkg=tor&arch=hurd-i386&ver=0.3.4.5-rc-1&stamp=1531586376&raw=0]

The appended patches replace tor_snprintf with tor_asprintf.

Trac:
Username: paulusASol