Few more tweaks to ref counting code

[Reviewed by David]

A few changes suggested by David after r22108:

- Add an integer argument to inc/decRefCount (default=1)
- Move error checking for negative ref count code into decRefCount
- Remove on statements added in a couple of autoCopy calls

Plus one other functional change:

- Removed check for x._value in chpl__autoCopy() for distributions



git-svn-id: http://svn.code.sf.net/p/chapel/code/trunk@22115 3a8e244f-b0f2-452b-bcba-4c88e055c3ca