Verify if nmdm(4) is available for creation of virtual ports.

Mention the dependency on the nmdm(4) kernel module in the
documentation.

man/wifibox.8

@@ -1,4 +1,4 @@
-.Dd August 6, 2021
+.Dd August 8, 2021
 .Dt WIFIBOX 8
 .Os
 .Sh NAME
@@ -135,10 +135,12 @@ kernel module and restarting the guest.
 .It Cm console
 Attach to the running guest with
 .Xr cu 1
-through a virtual serial port.  This is recommended for
-troubleshooting problems with the guest in an interactive fashion.  It
-can also be used to perform updates, install or remove packages.  This
-has to be configured specifically in order to work.
+through a virtual serial port, implemented by
+.Xr nmdm 4 .
+This is recommended for troubleshooting problems with the guest in an
+interactive fashion.  It can also be used to perform updates, install
+or remove packages.  This has to be configured specifically in order
+to work.
 .It Cm version
 Display version of
 .Nm
@@ -173,7 +175,8 @@ The exit status is 0 on success, and >0 if any of the commands fail.
 .Xr resolv.conf 5 ,
 .Xr devd.conf 5 ,
 .Xr devd 8 ,
-.Xr cu 1
+.Xr cu 1 ,
+.Xr nmdm 4
 .Sh CAVEATS
 .Nm
 supports only a single wireless network device at a time, and it has

sbin/wifibox

@@ -52,9 +52,11 @@ sysctl_value() {
     ${SYSCTL} -nq "$1"
 }
 
-check_vmm() {
-    if ! ${KLDSTAT} -q -m vmm; then
-	${ECHO} "ERROR: vmm.ko is not loaded."
+check_kmod() {
+    local _kmod="$1"
+
+    if ! ${KLDSTAT} -q -m ${_kmod}; then
+	${ECHO} "ERROR: ${_kmod}.ko is not loaded."
 	exit 127
     fi
 }
@@ -74,7 +76,7 @@ check_iommu() {
 }
 
 assert_vm_can_run() {
-    check_vmm
+    check_kmod "vmm"
     check_iommu
     check_virtfs
 }
@@ -111,6 +113,8 @@ create_nmdm() {
 
     [ "${console}" != "yes" ] && return 0
 
+    check_kmod "nmdm"
+
     [ ! -c ${NMDM_A} ] && ${MKNOD} ${NMDM_A}
     [ ! -c ${NMDM_B} ] && ${MKNOD} ${NMDM_B}
 }