02bc0a68 requires 24 bytes more on
64bit, which brought o00005.vtc just over the top on smartos 64bit
machines (the vtc tests PROXY TLVs, which get stored on the session
workspace).

Increase the default workspace_session by 256bytes (50%) to accomodate
sane proxy TLV lengths by default.

This is about 10x more than the minimum increase sufficient to
accomodate the increased workspace usage. I went for 768 bytes because

- this gives us the next least common multiple (12k) with the common 4K
  page size unless we went up to 1k

- we were already quite tight with the default and could not accomodate
  common proxy TLV use (see for example #3131)

As a ballpart reality check, I do not think that spending an additional
256MB should be an issue for users supporting 1M sessions.