Issue #11: Use rhizome_filehash_t everywhere
Remove default service=file for new manifests
Issue #11: improve types in file i/o functions

Use open(2)/read(2)/write(2) instead of fopen(3)/fread(3)/fwrite(3) in
several places to avoid unnecessary buffering

Fix a bug in Rhizome HTTP add's handling of unconfigured manifest
template file

Improve some debug and error logging for file i/o