mongofiles: support setting collection name

Currently mongofiles can only work on a collection named "fs" in the
given database. The collection argument is silently ignored.

This patch activates the collection argument allowing for an arbitrary
collection name, to which ".files" and ".chunks" are added as necessary.
mongofiles: allow setting of chunk size

The chunk size can now be defined on the command line when putting
documents into GridFS.
mongofiles: add re-chunk command to change chunk size of existing file

This is a convenience command that allows for changing the chunk size of
an existing GridFS file. It essentially performs a get followed by a put
using replace and a given chunk size. What it adds is the creation and
deletion of a temporary file inbetween.
mongofiles: do not re-chunk if chunk size is the same

If the old and new chunk size are the same skip the re-chunking step as
it would just be superfluous overhead. That is a small improvement that
pays of when resizing many documents in a row.