lib/util: Input validation in str_to_uint

This function returns true when 's' is negative or greater than UINT_MAX. Also,
the representation of 'int' and 'unsigned int' is implementation dependent, so
converting [INT_MAX..UINT_MAX] values with str_to_int is fragile.
Instead, we should convert straight to 'long long' and do a boundary check
before returning the converted value.
This patch also move the function to the .c file as it's not-trivial now, and
deletes the other str_to_u* functions as they are not used.

Signed-off-by: Zoltan Kiss <zoltan.kiss@citrix.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>