Fix PR 40151 - bootstrap of pkgsrc fails on Ubuntu 8.10
- At least Ubuntu 8.1 sets __attribute__((warn_unused_result)) on fwrite()
  http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25509
  which means (void)fwrite(...) will *always* generate a warning, so
  set -Wno-error to bypass this in Linux.sys.mk
- Ubuntu 8.1 also jumps through hoops to ensure ARG_MAX is *undefined*, so
  work around this in tnftp and libnbcompat

(abs)