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.

rc.d/anyenv

@@ -2,6 +2,6 @@
 
 # git clone https://github.com/riywo/anyenv ${HOME}/.anyenv
 
-if (which anyenv &>/dev/null); then
+if (command -v anyenv &>/dev/null); then
   eval "$(anyenv init -)"
 fi