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/cache.c

@@ -979,7 +979,7 @@ krb5_cc_get_prefix_ops(krb5_context context, const char *prefix)
 
     if (prefix == NULL)
 	return KRB5_DEFAULT_CCTYPE;
-    if (prefix[0] == '/')
+    if (ISPATHSEP(prefix[0]))
 	return &krb5_fcc_ops;
 
     p = strdup(prefix);