Using -Wshadow

GCC supports the -Wshadow command-line option:
      -Wshadow          Warn whenever a local variable shadows another local          variable, parameter or global
variableor whenever a          built-in function is shadowed.
 

Currently, enabling this for the PostgreSQL tree produces a lot of
warnings. Would anyone object if I corrected these usages of a
shadowed local variable, and then enabled this warning flag for
standard GCC builds? (as is currently done for -Wmissing-prototypes,
-Wmissing-declarations, and -Wall).

If there are any other GCC warning flags anyone else feels would be
useful, let me know.

-Neil