For Win-32: always #include windows.h

For Win-32: always #include windows.h

Change INTERFACE to be KRB5_CALLCONV, which is where the calling
convention is defined.  Add KRB5_DLLIMP which is where the DLL
import/export should be put for Win32.  (Win16 is just different.)
The correct way to declare a function which will be used in a DLL is
now: KRB5_DLLIMP func_return_t KRB5_CALLCONV func(long)

Change function delcarations to use the new convention.  Actually, it
doesn't hurt to use the old convention as long as func_return_t
doesn't contain a '*'.  But in the long run we should be exterminating
all uses of INTERFACE in favor of KRB5_CALLCONV and KRB5_DLLIMP.

git-svn-id: svn://anonsvn.mit.edu/krb5/trunk@8305 dc483132-0cff-0310-8789-dd5450dbe970