Re-enable statvfs() tests now that -CURRENT has the function.
Improve build reliability:
* Make very sure that the build script is run with /bin/sh.
* Add a dubious patch to libast/comp/conf.sh that is reported to fix
the build for some -STABLE users.
Install example functions.
PR: 39783, 40857
Parts
Submitted by: Joe Kelsey <joek@mail.flyingcroc.net>
-CURRENT has <sys/statvfs.h> but no actual statvfs() implementation,
which badly confuses the configure tests that check for this
functionality. Remove the tests for the time being.