tighten input to float4/float8/oid

This patch makes the following changes

- emit a warning when an empty string is input to the float4, float8,
and oid types, per earlier discussion on -hackers
- emit a warning when there is trailing whitespace in the input to the
oid type, per suggestion from Tom (BTW, I couldn't see anything about
this in the standard, although I may have been looking in the wrong
place; in any case, rejecting trailing whitespace seems clearly
correct behavior to me)
- update the regression tests

The first two changes are intended as a temporary measures for
backward compatibility: 7.5 will emit warnings, but 7.6 will reject
this input. That should hopefully avoid a similar situation to the
pg_atoi() change.

I invented a new SQLSTATE code for these warnings,
ERRCODE_WARNING_DEPRECATED_FEATURE. Did I do this correctly?

-Neil