For GASNet udp conduit + ssh spawner, send env to user prog via -E options.

This modifies our amudprun launcher to send all of the environment
variables to the user program instances via -E options.  Unfortunately,
doing so requires an update to GASNet, because the udp conduit's ssh
spawner can only handle 1024-byte command lines and the ones this mod
causes us to create are quite a lot larger than that.

The file-specific changes are as follows.

runtime/src/arg.c
Send a -E option to the user (*_real) program for every environment
setting.

third-party/gasnet/GASNet-1.22.0/other/amudp/amudp_spawn.cpp
Modify the udp conduit's ssh spawner to handl;e command lines of
arbitrary length.

third-party/gasnet/GASNet-1.22.0/gasnet_trace.c
Allow for much longer command lines in the tracing code.  I didn't do a
full solution for arbitrary-length command lines here, because command
line buffers with statrically determined lengths are everywhere in
GASNet and where would it all end?

test/execflags/gbt/getenv.good
test/execflags/gbt/getenv.prediff
Update the test that checks for correct functioning of this feature.