* environ.h (win_env::immediate): Declare new field.

* environ.h (win_env::immediate): Declare new field.
(win_env::operator = ): Declare new operator.
(win_env::reset): Declare new function.
(win_env::~win_env): Declare new destructor.
(getwinenv): Add optional third argument to declaration.
* environ.cc (conv_envvars): Accommodate immediate field.
(win_env::operator =): Define new operator.
(win_env::~win_env): Define new destructor.
(win_env::add_cache): Add value to environment immediately if "immediate" is
set.
(getwinenv): Accept optional third argument which will be used to store
"cached" values to avoid overwriting real cache.
(spenv::force): Declare new field.
(spenvs): Accommodate force field.  Add "PATH=" with force set to true.
(spenv::retrieve): Avoid duping anything if we're not building an envblock.
(build_env): Ditto.  Use size of potentially constructed new environment block
to determine if we need to create an environment block.  Pass getwinenv
temporary storage to avoid inappropriately overwriting the environment cache.