Change some test macros to return true booleans

These macros work fine when they are used directly in an "if" test or
similar, but as soon as the return values are assigned to boolean
variables (or passed as boolean arguments to some function), they become
bugs, hopefully caught by compiler warnings.  To avoid future problems,
fix the definitions so that they return actual booleans.

To further minimize the risk that somebody uses them in back-patched
fixes that only work correctly in branches starting from the current
master and not in old ones, back-patch the change to supported branches
as appropriate.

See also commit af4472bcb88ab36b9abbe7fd5858e570a65a2d1a, and the long
discussion (and larger patch) in the thread mentioned in its commit
message.

Discussion: https://postgr.es/m/18672.1483022414@sss.pgh.pa.us

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