Use posix_fallocate() for new WAL files, where available.

This function is more efficient than actually writing out zeroes to
the new file, per microbenchmarks by Jon Nelson. Also, it may reduce
the likelihood of WAL file fragmentation.

Jon Nelson, with review by Andres Freund, Greg Smith and me.
Update messages, comments and documentation for materialized views.

All instances of the verbiage lagging the code.  Back-patch to 9.3,
where materialized views were introduced.