ldb: baseinfo pack format check on init

ldb: baseinfo pack format check on init

We will be adding a new packing format in forthcoming commits and there
may be more versions in the future. We need to make sure the database
contains records in a format we know how to read and write.
Done by fetching the @BASEINFO record and reading the first 4
bytes which contain the packing format version.

NOTE: Configure with --abi-check-disable to build this commit. This
patch is part of a set of LDB ABI changes, and the version update is
done on the last commit.

Signed-off-by: Aaron Haslett <aaronhaslett@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Gary Lockyer <gary@catalyst.net.nz>