Refactor space allocation for base64 encoding/decoding in pgcrypto.

Refactor space allocation for base64 encoding/decoding in pgcrypto.

Instead of trying to accurately calculate the space needed, use a StringInfo
that's enlarged as needed. This is just moving things around currently - the
old code was not wrong - but this is in preparation for a patch that adds
support for extra armor headers, and would make the space calculation more
complicated.

Marko Tiikkaja