Increase fixed filename sizes in SLIP tunnels

Long serial port names like
  /dev/serial/by-id/usb-FTDI_FT230X_Basic_UART_DN0038W8-if00-port0
cause crashes.  This is the simplest fix to avoid the problem.
Merge pull request #737 from jimparis/slip-buffer-overflow-fixes

Increase fixed filename sizes in SLIP tunnels