tcpwrapper: Fix signage problems in the tcp_wrappers hash function

Eliminate the following compiler warnings:

tcpwrapper.c:78: warning: no previous prototype for strtoint
tcpwrapper.c: In function strtoint
tcpwrapper.c:81: warning: conversion to int size_t may change the
    sign of the result
tcpwrapper.c:85: warning: conversion to unsigned int from int may
    change the sign of the result
tcpwrapper.c: In function hashint:
tcpwrapper.c:91: warning: conversion to int from unsigned int may
    change the sign of the result

The hash value is probably computed consistently even with
unexpected sign inversions.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>