configure fixes

a+=b is not supported by all shells. It is equivalent to a=${a}b, so
just rewrite it as that.

This also fixes commit 9ea6c3d3, which intended to only make the
definitions of _dracutdir, _udevdir, and _udevruledir conditional, but
actually ensured that _initconfdir no longer got defined if _dracutdir
was defined, and defined _udevdir to the value that should have been
used for _udevruledir.

This also fixes the fact that the checks introduced by commit 9ea6c3d3
could never work: ZFS_AC_PACKAGE was called before the configuration
options were processed.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Harald van Dijk <harald@gigawatt.nl>
Closes #10518