getenv() after my_setenv() gets old entry on Win32

getenv() after my_setenv() gets old entry on Win32

Perl uses the environment to communicate the -d:DProf
switch to itself.

Since the win32 code sets the operating system's env block
directly when my_setenv() is called, calls to the RTL's
getenv() won't see the changed environment, so the -d:Foo
option is not honored.

The attached patch fixes the problem by supplying our own
getenv() equivalent.

p5p-msgid: 199706231700.NAA23400@aatma.engin.umich.edu