Proposed filename length check for SPIFFS

[sticilface]

as mentioned #1088

[igrr]

• I think this check should be done not it FS abstraction layer, but in SPIFFSImpl. Reason is, we may have another FS implementation (like SD) which doesn't share this limitation.
• This check should also be done in all other functions which take file names. Perhaps it's worth adding a function like bool SPIFFSImpl::_isValidFilename(const char*) which would be called from other places.

[sticilface]

This sounds very reasonable

[sticilface]

Looking at https://github.com/esp8266/Arduino/blob/master/cores/esp8266/spiffs_api.cpp#L49 it might be better for you to implement it. I don't understand the code that well!