Use the explicit_bzero() function in places where it is important that
security information such as passwords is cleared from memory.  There
might be other places where it could be useful; this is just an
initial collection.

For platforms that don't have explicit_bzero(), provide various
fallback implementations.  (explicit_bzero() itself isn't standard,
but as Linux/glibc, FreeBSD, and OpenBSD have it, it's the most common
spelling, so it makes sense to make that the invocation point.)

Discussion: https://www.postgresql.org/message-id/flat/42d26bde-5d5b-c90d-87ae-6cab875f73be%402ndquadrant.com