style: Use command -v instead of which for availability checks

rc.d/anyenv, rc.d/screen, aliases.d/mc, and rc.d/startx were the last
holdouts still using `which` to check tool availability. Every other
file in rc.d/* already uses `command -v`, which doesn't depend on an
external binary being installed.

aliases.d/mc

@@ -1,4 +1,4 @@
-if (which mc &> /dev/null); then
+if (command -v mc &> /dev/null); then
   if [ "${TERM}" = "rxvt-unicode" ]; then
     alias mc="TERM=rxvt-unicode-256color mc"
   fi