Make kprop work for dump files larger than 4GB

Make kprop work for dump files larger than 4GB

If the dump file size does not fit in 32 bits, encode four zero bytes
(forcing an error for unmodified kpropd) followed by the size in the
next 64 bits.

Add a functional test case, but only run it when an environment
variable is set, as processing a 4GB dump file is too
resource-intensive for make check.

[ghudson@mit.edu: edited comments and commit message; eliminated use
of defined constant in some cases; added test case]

ticket: 9053 (new)