lib/tdb_compat: header for tdb1 vs tdb2.

lib/tdb_compat: header for tdb1 vs tdb2.

TDB2's API is slightly different from TDB1.  In particular, all functions
return 0 (TDB_SUCCESS) or a negative error number, rather than -1 or tdb_null
and storing the error in tdb_error() (though TDB2 does that as well).

The simplest fix is to replace all the different functions with a wrapper,
and that is done here.

Compatibility functions:

tdb_null: not used as an error return, so not defined by tdb2.
tdb_fetch_compat: TDB1-style data-returning tdb_fetch.
tdb_firstkey_compat: TDB1-style data-returning tdb_firstkey
tdb_nextkey_compat: TDB1-style data-returning tdb_nextkey, with
	TDB2-style free of old key.
tdb_errorstr_compat: TDB1-style tdb_errorstr() which takes TDB instead of ecode.
TDB_CONTEXT: TDB1-style typedef for struct tdb_context.
tdb_open_compat: Simplified open routine which takes log function, sets
	TDB_ALLOW_NESTING as Samba expects, and adds TDB_CLEAR_IF_FIRST support.

Things defined away in TDB2 wrappers:

tdb_traverse_read: TDB2's tdb_traverse only uses read-locks anyway.
tdb_reopen/tdb_reopen_all: TDB2 detects this error itself.
TDB_INCOMPATIBLE_HASH: TDB2 uses the Jenkins hash already.
TDB_VOLATILE: TDB2 shouldn't have freelist scaling issues.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>