Instead of using a signed type (`off_t`) use a new `git_object_size_t`
for the sizes of objects.

Using an `enum` causes trouble when used with C++ as bitwise operations are not possible w/o casting (e.g., `opts.flags &= ~GIT_BLOB_FILTER_CHECK_FOR_BINARY;` is invalid as there is no `&=` operator for `enum`).

Signed-off-by: Sven Strickroth <email@cs-ware.de>

When `GIT_BLOB_FILTER_ATTTRIBUTES_FROM_HEAD` is passed to
`git_blob_filter`, read attributes from `gitattributes` files that
are checked in to the repository at the HEAD revision.  This passes
the flag `GIT_FILTER_ATTRIBUTES_FROM_HEAD` to the filter functions.

Introduce `GIT_BLOB_FILTER_NO_SYSTEM_ATTRIBUTES`, which tells
`git_blob_filter` to ignore the system-wide attributes file, usually
`/etc/gitattributes`.

This simply passes the appropriate flag to the attribute loading code.

Users should now use `git_blob_filter`.

Provide a function to filter blobs that allows for more functionality
than the existing `git_blob_filtered_content` function.

The majority of functions are named `from_something` (with an
underscore) instead of `fromsomething`.  Update the blob functions for
consistency with the rest of the library.

Signed-off-by: Sven Strickroth <email@cs-ware.de>