Various small fixes

[laurentg]

• Correct polyline encoding bug for small negative delta (when float c is negative but rounded binary is 0, ~0 = FFFF...)
• Polyline encoding: use proper rounding for float to binary conversion, this allow polyline tests copied over from the web to pass properly (also correct few previous tests)
• Add distance to walk legs in JSON output, as it seems to be mandatory for existing OTP clients
• Remove duplicated initializations in request buffer, zero out the buffer to zero the padding bytes and to make it safer in case somebody add new fields
• Output NULL instead of "null" string for plan error when no error to make JSON output compatible with OTP
• Replace inexisting (removed?) function in hashgrid unit function.

[koch-t]

Sorry, this was a side-effect from a recovery needed after a @skinkie cock-up