common: Add new function b64decode.
* common/b64dec.c (b64decode): New. * common/t-b64.c: Change license to LGPL. (oops): New macro. (hex2buffer): New. (test_b64decode): New. (main): Default to run the new test. * common/Makefile.am (module_maint_tests): Move t-b64 to ... (module_tests): here.
Sometimes we have a short base64 encoded string we need todecode.
This function makes it simpler.
License change of the test module justified because I am the single
author of the code.