buildcache doesn't track dup, dup2, dup3

[lstewart]

discovered by noticing that make test1 fails on systems which have an /etc/motd

test1 uses sort(1) as an exemplar build command. At least one version of sort does a
dup2(3,1) to set its output file descriptor to 1

buildcache tracks the write to this output file, but the file decriptor is unknown, so buildcache skips it (silently).

Either a write to an unknown fd should be an error, or (better) buildcache should track dup, dup2, dup3

Generally this seems straightforward, we add suitable regular expressions to buildcache to decode the dup system calls, and copy the open file information to the new fd.