Ensure that btree sort ordering functions and boolean comparison operators

Ensure that btree sort ordering functions and boolean comparison operators
give consistent results for all datatypes.  Types float4, float8, and
numeric were broken for NaN values; abstime, timestamp, and interval
were broken for INVALID values; timetz was just plain broken (some
possible pairs of values were neither < nor = nor >).  Also clean up
text, bpchar, varchar, and bit/varbit to eliminate duplicate code and
thereby reduce the probability of similar inconsistencies arising in
the future.