* environ.cc: Use new definition of "environ" throughout.

* environ.cc: Use new definition of "environ" throughout.
(environ_init): Explicitly initialize __cygwin_environ.
(cur_environ): New function.  Detects when user has updated their environment.
* exec.cc: Use 'environ' define throughout rather than __cygwin_environ.
* spawn.cc: Ditto.
* winsup.h: Declare cur_environ, main_environ, environ.