databases/ldb28: Add a new version of ldb

This port is based on ldb25. The changes are:

• Fix checks for Python support
  
  The bsd.port.pre.mk was included too early and USES=python wasn't processed as expected. As a result, the checks for Python support were broken.

• Switch from waf to configure and make.
  
  Upstream wants us to do it this way. Otherwise, the build system produces the following error message:
  
  > ===> Configuring for ldb28-2.8.1 > PYTHONHASHSEED=1 missing! Don't use waf directly, use ./configure and make!
  
  So, set HAS_CONFIGURE and BINARY_ALIAS to make the port build without waf.

• Clean up the makefile (sort variables and fix indentation)

• Drop the NO_PYTHON variable. It complicates the makefile unnecessarily. It is still possible to disable Python support by configuring the PYTHON3 option.

• Use WITH_DEBUG instead of option DEBUG.

• Enable PYTHON3 by default.
  
  The PYTHON3 option is required by net/samba419 to build without the bundled dependencies. The primary purpose of databases/ldb28 is to make it possible, so enable Python support by default.

PR: 280510
Sponsored by: Klara, Inc.