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/keytab.c

@@ -170,7 +170,7 @@ keytab_name(const char *name, const char **type, size_t *type_len)
     residual = strchr(name, ':');
 
     if (residual == NULL ||
-	name[0] == '/'
+	ISPATHSEP(name[0])
 #ifdef _WIN32
         /* Avoid treating <drive>:<path> as a keytab type
          * specification */