homework: Handle Update & Create w/ blob dir

homework: Handle Update & Create w/ blob dir

Introduces new extended variants of the various incarnations of
Create and Update, which take a map of filenames to FDs. This map is
then used to populate the bulk directory.

FDs are used to prevent the client from abusing homed's blob directory
permissions (everything is made world-readable by homed) to open files
that they normally aren't allowed to open. Passing along an FD ensures
that the client has read access to the file it wants homed to make
world-readable.

Internally, homework uses the map to overwrite the system blob dir.
Later, homework's existing blob dir reconciliation logic will propagate
the new contents from the system blob dir into the embedded blob
dir