gssapi: support for client keytab in gss_acquire_cred (#383)

For compatibility with MIT Kerberos, support automatic acquisition of initiator
credentials if a client keytab is available. The default path on non-Windows is
/var/heimdal/user/%{euid}/client.keytab, but can be overriden with the
KRB5_CLIENT_KTNAME environment variable or the default_client_keytab_name
configuration option. If a client keytab does not exist, or exists but does not
contain the principal for which initiator credentials are being acquired, the
system keytab is tried.

include/config.h.w32

@@ -1450,6 +1450,9 @@ static const char *const rcsid[] = { (const char *)rcsid, "@(#)" msg }
 
 #define LIBDIR "%{LIBDIR}"
 
+/* For compatibility with MIT, {USERCONFIG} would be better */
+#define CLIENT_KEYTAB_DEFAULT   "FILE:%{WINDOWS}\\krb5clientkt"
+
 #endif	/* RC_INVOKED */