Various fixes, mainly to cut down on compiler warnings

- Fix various format specifications that didn't match argument types
- Remove explicit '\0' at end of format strings
- Remove several unused variables
- Fix buffer overflow in telnetServer when at least 10 TELNET devices configured
- Make _doprnt() (called by printf(), sprintf(), fprintf()) slightly more
  standards-compliant:
    - Allow flags to be specified in any order
    - Add support for length modifiers
    - Handle negative minimum field width correctly
    - Avoid undefined negation of signed long with value LONG_MIN
    - Handle integer precision correctly
- Don't attempt to compile xsh_ethstat.c, xsh_gpiostat.c, xsh_led.c when not
  supported in configuration
- Detect invalid delay passed to 'sleep' command