webapi: don't accept zero-length childnames during traversal. Closes #358, #676.

webapi: don't accept zero-length childnames during traversal. Closes #358, #676.

This forbids operations that would implicitly create a directory with a
zero-length (empty string) name, like what you'd get if you did "tahoe put
local /oops/blah" (#358) or "POST /uri/CAP//?t=mkdir" (#676). The error
message is fairly friendly too.

Also added code to "tahoe put" to catch this error beforehand and suggest the
correct syntax (i.e. without the leading slash).