Add stdio-like I/O interfaces to the test_quota VFS. This is a prototype

Add stdio-like I/O interfaces to the test_quota VFS.  This is a prototype
change for discussion and is mostly untested.  This is an alternative to
adding stdio-like I/O interfaces in the core.  There is no guarantee that
this code will make it into the trunk.  If it does get to trunk, there
could be many changes to the interface first.

FossilOrigin-Name: bd3ce723f1b5be52be46ede8614ca316f56e7e6f