timeout(1): fix the acceptable range values for parse_signal()

[alex_kaworu.ch]

Summary

Before this patch, both 0 and sys_nsig would be successfully returned by parse_signal() although being invalid signal numbers.

Test Plan

none

[alex_kaworu.ch]

strtonum(3) says that minval and maxval are inclusive:

      int iterations;
      const char *errstr;

      iterations = strtonum(optarg, 1, 64, &errstr);
      if (errstr != NULL)
              errx(1, "number of iterations is %s: %s", errstr, optarg);

The above example will guarantee that the value of iterations is between
1 and 64 (inclusive).