Revise int2/int4/int8/float4/float8 input routines to allow for

any amount of leading or trailing whitespace (where "whitespace"
is defined by isspace()). This is for SQL conformance, as well
as consistency with other numeric types (e.g. oid, numeric).

Also refactor pg_atoi() to avoid looking at errno where not
necessary, and add a bunch of regression tests for the input
to these types.