Support case insensitive treatment of os.environ keys on Windows and

Support case insensitive treatment of os.environ keys on Windows and
DOS (as well as OS/2).  I presume that making a call to putenv() with
a lowercase key will actually do the right thing.  I know this is so
on Windows/DOS, and I expect it is so OS/2 -- but the old OS/2 code
didn't assume this.  (I don't know if the person who provided the OS/2
patch was clueless or just didn't care about DOS and Windows.)

Also ripped out the support for pickling -- as of 1.5, this is no
longer needed to make pickling work.