* swarm, cmd/swarm: address ineffectual assignments

* swarm/network: remove unused vars from testHandshake

* swarm/storage/feed: revert cursor changes

These minor changes already shaved off around 30s.

- Mime types generator (Standard "mime" package rely on system-settings, see mime.osInitMime)
- Changed swarm/api.Upload:
    - simplify I/O throttling by semaphore primitive and use file name where possible
    - f.Close() must be called in Defer - otherwise panic or future added early return will cause leak of file descriptors
    - one error was suppressed

* cmd/swarm: fix manifest subcommands and add tests

* cmd/swarm: manifest update: update default entry for non-encrypted uploads

* swarm/api: upload defaultpath file only once

* swarm/api/client: improve UploadDirectory default path handling

* cmd/swarm: support absolute and relative default path values

* cmd/swarm: fix a typo in test

* cmd/swarm: check encrypted uploads in manifest update tests

Previously, NewManifest was asynchronous so subsequent code which tried
to use the returned manifest could error as the manifest was not yet
persisted.