www: preload: load all encodings at startup

www: preload: load all encodings at startup

Encode lazy-loads encodings on an as-needed basis.  This is
great for short-lived programs, but leads to fragmentation in
long-lived daemons where immortal allocations can get
interleaved with short-lived, per-request allocations.

Since we have no idea which encodings will be needed when
there's a constant flow of incoming mail, just preload
everything available at startup.