Add an option for debugserver to propagate its environment to programs it...

Add an option for debugserver to propagate its environment to programs it launches using the --forward-env or -F:

% ./debugserver --forward-env localhost:1234 -- /bin/ls
% ./debugserver -F localhost:1234 -- /bin/ls

Also allow new environment variables to be set using the "--env" or "-e":

% ./debugserver --env FOO=1 --env BAR=2 localhost:1234 -- /bin/ls
% ./debugserver -e FOO=1 -e BAR=2 localhost:1234 -- /bin/ls

<rdar://problem/17350654> 

llvm-svn: 211200