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

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

rc.d/screen

@@ -1,5 +1,5 @@
 # GNU Screen stuff
-if ( which screen &> /dev/null )
+if ( command -v screen &> /dev/null )
 then
   if [[ ${TERM} != screen* ]]
   then

rc.d/startx

@@ -13,12 +13,12 @@ if [[ -z "${DISPLAY}" && $(id -u) -ge 1000 ]] ; then
             3) VT=vt3 ;;
         esac
 
-        if ( which ck-launch-session &> /dev/null )
+        if ( command -v ck-launch-session &> /dev/null )
         then
             EXEC="exec ck-launch-session"
         fi
 
-        if ( which startx &> /dev/null )
+        if ( command -v startx &> /dev/null )
         then
             ${EXEC} startx 1> ~/.log/xsession-errors 2>&1 -- "${VT}"
         fi