Add disable-luks2 reencryption configure option.

The option --disable-luks2-reencryption completely disable
LUKS2 reencryption code.

When used, the libcryptsetup library can read metadata with
reencryption code, but all reencryption API calls and cryptsetup
reencrypt commands are disabled.

Devices with online reencryption in progress cannot be activated.

This option can cause some incompatibilities. Please use with care.

lib/luks2/luks2_keyslot.c

@@ -27,7 +27,9 @@ extern const keyslot_handler reenc_keyslot;
 
 static const keyslot_handler *keyslot_handlers[LUKS2_KEYSLOTS_MAX] = {
 	&luks2_keyslot,
+#if USE_LUKS2_REENCRYPTION
 	&reenc_keyslot,
+#endif
 	NULL
 };