ifconfig: Fix the display of the IFCAP_MEXTPG capability

Historically this capability is IFCAP_NOMAP but it was renamed to
IFCAP_MEXTPG. Catch up with the change 3f43ada98c89.

This is a direct commit to stable/13.

PR: 289545
Fixes: 3f43ada98c89 Catch up with 6edfd179c86: mechanically rename IFCAP_NOMAP to IFCAP_MEXTPG