Enhance the VACUUM INTO command such that if the target file is a URI

Enhance the VACUUM INTO command such that if the target file is a URI
with a reserve=N query parameter, then the reserve_bytes value is set to
N provided that N is within the allowed range.  This can even reduce the
size of the reserve, all the way to zero.  This provides a way to remove
reserve from a database without having to do a complete dump/restore.

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