vfs: Buffer names for nul termination

[chrysn]

This fixes a severe bug where file names were passed to the C API that expects nul-termination.

This incurs some copying overhead, which fortunately is not too bad due to the name length limit.

Closes: #93

Thanks @maikerlab for reporting this. AIU it went unnoticed because in my tests, the &str often came from what was originally nul-terminated, eg. from the command line, or from CoAP where the option byte is sometimes replaced with a \0 as a part gcoap's internal processing.

[chrysn]

As a follow-up, one could provide equivalent functions that pass in a cstr -- that would allow zero-overhead use, but I'm unsure whether people actually have their hands on such strings.