grow cmd_buff size to allow longer BinVoucher cmdline and parameters

as it was, there was hardly enough chars even for the client's mac argument,
which would get cut in half when the binary was called.

Signed-off-by: Gui Iribarren <gui@altermundi.net>
Merge pull request #15 from libre-mesh/master

grow cmd_buff size to allow longer BinVoucher cmdline and parameters