Allow 'Infinity' and '-Infinity' as input to the float4 and float8

Allow 'Infinity' and '-Infinity' as input to the float4 and float8
types. Update the regression tests and the documentation to reflect
this. Remove the UNSAFE_FLOATS #ifdef.

This is only half the story: we still unconditionally reject
floating point operations that result in +/- infinity. See
recent thread on -hackers for more information.