SF patch #460805 by Chris Gonnerman: Support for unsetenv()

This adds unsetenv to posix, and uses it in the __delitem__ method of
os.environ.

(XXX Should we change the preferred name for putenv to setenv, for
consistency?)
diff --git a/configure b/configure
index d85057d..4b71c3b 100755
--- a/configure
+++ b/configure
@@ -1,6 +1,6 @@
 #! /bin/sh
 
-# From configure.in Revision: 1.273 
+# From configure.in Revision: 1.274 
 
 # Guess values for system-dependent variables and create Makefiles.
 # Generated automatically using autoconf version 2.13 
@@ -4824,7 +4824,7 @@
  setlocale setregid setreuid setsid setpgid setuid setvbuf snprintf \
  sigaction siginterrupt sigrelse strftime strptime symlink sysconf \
  tcgetpgrp tcsetpgrp tempnam timegm times tmpfile tmpnam tmpnam_r \
- truncate uname waitpid _getpty getpriority
+ truncate uname unsetenv waitpid _getpty getpriority
 do
 echo $ac_n "checking for $ac_func""... $ac_c" 1>&6
 echo "configure:4831: checking for $ac_func" >&5