ISPATHSEP and ISTILDE macros; Windows portability

Windows treats '\\' and '/' equivalently but we cannot control
the form that will be used by end users.  Introduce ISPATHSEP()
macro which tests only for '/' on UNIX and both on Windows.
Introduce ISTILDE() macro to test for '~'.   When testing for
'/' with strchr() or strrchr() add conditional checks for '\\' on
Windows.

Change-Id: Ia85e698fc88f15a6a71db649db5417f02ef7e5fe

lib/krb5/plugin.c

@@ -241,6 +241,10 @@ resolve_origin(const char *di)
         return strdup(LIBDIR "/plugin/krb5");
 
     dname = dl_info.dli_fname;
+#ifdef _WIN32
+    p = strrchr(dname, '\\');
+    if (p == NULL)
+#endif
     p = strrchr(dname, '/');
     if (p)
         *p = '\0';