Bugs all spotted by Coverity, including wrong realloc() size request
and memory leaks. Cosmetic improvements by me.
The usage of the global variable "filemap" here is still pretty awful,
but at least I got rid of the gratuitous aliasing in several routines
(which was helping to annoy Coverity, as well as being a bug risk).
Earlier versions of this tool were available (and still are) on github.
Thanks to Michael Paquier, Alvaro Herrera, Peter Eisentraut, Amit Kapila,
and Satoshi Nagayasu for review.