* 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.

winsup/cygwin/environ.h

@@ -27,13 +27,17 @@ struct win_env
     int (*towin32) (const char *, char *);
     int (*posix_len) (const char *);
     int (*win32_len) (const char *);
+    bool immediate;
     void add_cache (const char *in_posix, const char *in_native = NULL)
       __attribute__ ((regparm (3)));
     const char * get_native () const {return native ? native + namelen : NULL;}
     const char * get_posix () const {return posix ? posix : NULL;}
+    struct win_env& operator = (struct win_env& x);
+    void reset () {native = posix = NULL;}
+    ~win_env ();
   };
 
-win_env * __stdcall getwinenv (const char *name, const char *posix = NULL)
+win_env * __stdcall getwinenv (const char *name, const char *posix = NULL, win_env * = NULL)
   __attribute__ ((regparm (3)));
 char * __stdcall getwinenveq (const char *name, size_t len, int)
   __attribute__ ((regparm (3)));