* environ.cc (spenv::retrieve): Detect return of env_dontadd from cygheap_user

* environ.cc (spenv::retrieve): Detect return of env_dontadd from cygheap_user
methods.
(build_env): Avoid incrementing environment pointer if not actually adding to
the environment.  That could result in garbage in the environment table.  Be
more defensive when reallocing envblock.