We need to have at least an internal wipememory function in GpgRT. But it makes also sense to have this in the public API.

This is required to implement an es_fopen mode flag "wipe", which runs wipememory as part of es_fclose. Right now we need to resort to es_fclose_snatch and do a wipememory on the returned buffer. This should work if es_fopenmem_init was used but is not very elegant.

This does not need to replace the speed optimized wipememory from Libgcrypt. It should replace the wipememory we have in gnupg, though.