Fix strn_to_sid_t(), add strn_to_rhizome_bk_t()

Improve strn_to_rhizome_bid_t() and strn_to_rhizome_filehash_t() to
accept a NULL first arg -- useful for data validation
Remove some unconditional debug from HTTP server
Log error in "rhizome extract" if payload fails
More Rhizome external blob debug
Fix rhizome_fill_manifest() 'name' semantics

Only set 'name' field if the given pathname is valid

Do not set 'name' field if no pathname supplied
UNFINISHED Implement HTTP POST /restful/rhizome/insert