loader: add nvidia modules to the loader blacklist

It is known that nvidia modules, like the drm modules that are already
present, will panic if loaded in early boot rather than later.  Pop them
into our list to deny loading them if someone were to add, e.g.,
nvidia-modeset_load="YES", to their loader.conf.  This doesn't prevent
them from being loaded if one drops to the loader prompt, but it does
prevent the standard user from easy foot-shooting if they find old or
inaccurate information out in the wild.

Reviewed by:	imp, kbowling

(cherry picked from commit 763a097c15)

stand/defaults/loader.conf

@@ -111,6 +111,7 @@ kernels_autodetect="YES"	# Auto-detect kernel directories in /boot
 #currdev="disk1s1a"		# Set the current device
 module_path="/boot/modules;/boot/dtb;/boot/dtb/overlays"	# Set the module search path
 module_blacklist="drm drm2 radeonkms i915kms amdgpu if_iwlwifi if_rtw88 if_rtw89"	# Loader module blacklist
+module_blacklist="${module_blacklist} nvidia nvidia-drm nvidia-modeset"
 #prompt="\\${interpret}"	# Set the command prompt
 #root_disk_unit="0"		# Force the root disk unit number
 #rootdev="disk1s1a"		# Set the root filesystem