Use readarray in bash v4+ to avoid rbenv init hanging

For just a handful of people, rbenv init would hang indefinitely. Turning on
debugging output suggested that the `read` expression to split PATH into a bash
array was hanging, but I could never reproduce this myself. Instead, this uses
bash v4+ `readarray` if it's available, or falls back to bash v3 `read` with the
default DELIM being a newline character. This will cause a regression if any PATH
entries contain a literal newline character, but hopefully people are not relying
on such paths.

libexec/rbenv-commands

@@ -26,7 +26,12 @@ elif [ "$1" = "--no-sh" ]; then
   shift
 fi
 
-IFS=: read -d '' -r -a paths <<<"$PATH" || true
+if [ "$(type -t readarray)" = "builtin" ]; then
+  readarray -d : -t paths < <(printf "%s" "$PATH")
+else
+  # bash 3.x compatibility
+  IFS=: read -r -a paths <<<"$PATH" || true
+fi
 
 shopt -s nullglob