Move _putenv and _wputenv out of _POSIX_ guard.

See http://msdn.microsoft.com/en-us/library/83zh4e6k.aspx
for more details.
Move _wspawn functions from process.h to stdio.h and wchar.h.

See http://msdn.microsoft.com/en-us/library/20y988d2.aspx
for more details.
Functions _spawnv, _spawnve, _spawnvp and _spawnvpe should be decared in both stdio.h and process.h.

See http://msdn.microsoft.com/en-us/library/20y988d2.aspx
for more details.