#9907: call rl_initialize early when using editline on OSX

editline rl_initialize apparently discards any mappings done before it
is called, which makes tab revert to file completion instead of inserting
a tab.  So now on OSX we call rl_initialize first if we are using
readline, and then re-read the users .editrc (if any) afterward so they
can still override our defaults.

Patch by Ned Deily, modified by Ronald Oussoren.