With GASNet amudprun+ssh launch, forward environment to user process.

GASNet's amudprun launcher does not forward the environment from the
launcher to the the user processes.  Do it ourselves in this case, using
the new -E option to the user program.

runtime/src/launch/amudprun/launch-amudprun.c
  If we using the ssh spawner, add the environment to the user program
  command line via the new -E option.
  While here, I also did a little cleaning up: moved _nlbuf (renamed to
  nlbuf, what's with the leading '_' on a file-static var?) into the
  only function that uses it, and changed sprintf(3) to snprintf(3) in
  the two places where we print into limited-size buffers.  The buffers
  should always be long enough, but it's still good style.  I also got
  rid of a pair of comments that didn't seem to be adding anything.

test/execflags/gbt/getenv.chpl
test/execflags/gbt/getenv.execenv
test/execflags/gbt/getenv.good
test/execflags/gbt/getenv.numlocales
test/execflags/gbt/getenv.skipif
  Add a test that runs only for comm=gasnet, and checks that a user
  environment variable is available.
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.
Put the GASNet source files back how they were. I'm going to apply the
official GASNet version of the changes to those files instead.
Remove arbitrary limit on command line length

This commit is a minor variation on one contributed by Greg Titus
of Cray.  The purpose is to remove the arbitrary 1024-character
limit on the length of the ssh command line run via system() when
AMUDP is using SPAWNFN=S.  This allows for a much longer command
line, as required by the Chapel runtime when using this spawner.
Merge pull request #194 from gbtitus/amudprun-forward-env

with GASNet udp conduit and ssh spawning, forward the environment

[Reviewed by Sung.]

GASNet's udp conduit and ssh spawner do not forward the environment from
the launching process to the user processes. We therefore need to do it
ourselves. To achieve this, in our amudprun launcher add a -E option
for each environment variable to the command line for the user program.

runtime/src/launch/amudprun/launch-amudprun.c
If we are using the ssh spawner, add the environment to the user
program command line via the new -E option.

While here, I also did a little cleaning up. I moved nlbuf into the
only function that uses it. While doing so I renamed _nlbuf to nlbuf;
the leading '' seemed extraneous. I also changed sprintf(3) to
snprintf(3) in the two places where we print into limited-size
buffers. The buffers should always be long enough, but it's still
good style. And, I got rid of a pair of comments that didn't seem to
be adding anything.

runtime/src/arg.c
Don't print a warning when we see a -E option for an env var that is
already set. It turns out there are lots of these, set by various
things that run before the user executable in the remote ssh session.
The warnings are therefore unavoidable and just add noise.

third-party/gasnet/GASNet-1.22.0/other/amudp/amudp_spawn.cpp
Allow command lines of arbitrary length in the udp conduit's ssh
spawner. The old code only allowed 1024 characters, used a fixed
size buffer, and didn't check for overflow. Adding the entire
environment to the command line typically requires way more than 1024
characters. The change here duplicates this GASNet commit:
https://bitbucket.org/berkeleylab/gasnet/commits/1abde2638f58790b6e649ad23d\
42a19a6b3166d3

This is a minor modification of some code I sent Paul Hargrove. To
ease future merges, I'm committing his version instead of my original.
(I committed and then backed out mine in my branch, so anyone curious
can dig it up.)

test/execflags/gbt/getenv.chpl
test/execflags/gbt/getenv.execenv
test/execflags/gbt/getenv.good
test/execflags/gbt/getenv.numlocales
test/execflags/gbt/getenv.prediff
test/execflags/gbt/getenv.skipif
Add a test that runs only for comm=gasnet, and checks that a user
environment variable is available.