The common.sh script assumed that it was either being run from
in-tree or was installed under /usr/libexec/zfs. If this was
not the case, because of say the default --prefix=/usr/local,
then the paths would be wrong. To fix this common.sh is now
generated from common.sh.in with the correct path information
provided at configure time.
This change extends the existing in-tree test infrastructure such
that it can also be run as part of a the installed package. This
simplifies testing on multiple systems and is generally all around
useful. The scripts may still be run in-tree and will use the
in-tree build products as long as .script-config exists.