Change do_source(), allowing . & source to succeed

Previously, invoking . or source in the following context would fail:

	% osh -c '. file [arg1 ...]' [arg1 ...]

or:

	% osh -c 'source file [arg1 ...]' [arg1 ...]

...

The shell would always read an EOF, resulting in a noop.  Now, it
succeeds as expected.