make HTTPFile safe for reading from multiple goroutines.

update zip so if you Close its reader before reading everything (e.g.,
because the end of a source file isn't used), we read the rest of the input
to a blackhole so the decompressor doesn't complain about a broken
pipe. and wire up Close() through the rest of dltp.

you could handle closing zips other ways, like just silencing decompressor's
error messages after Close.  this seems good for current use case, though.
Merge branch 'master' of https://github.com/twotwotwo/dltp