Avoid comparisons of different signs

Avoid comparisons of different signs

Comparisons if different signedness can result in unexpected results.
Add casts to ensure operants are of the same type.

    gettime.c: In function 'gettime':
    gettime.c:58:26: warning: comparison of integer expressions of different signedness: 'long long unsigned int' and 'time_t' {aka 'long int'} [-Wsign-compare]
       58 |         } else if (epoch > fallback) {
          |                          ^

Cast to time_t, since epoch is less than ULONG_MAX at this point.

    idmapping.c: In function 'write_mapping':
    idmapping.c:202:48: warning: comparison of integer expressions of different signedness: 'int' and 'long unsigned int' [-Wsign-compare]
      202 |                 if ((written <= 0) || (written >= (bufsize - (pos - buf)))) {
          |                                                ^~

    newgidmap.c: In function ‘main’:
    newgidmap.c:178:40: warning: comparison of integer expressions of different signedness: ‘int’ and ‘long unsigned int’ [-Wsign-compare]
      178 |         if ((written <= 0) || (written >= sizeof(proc_dir_name))) {
          |                                        ^~
    newuidmap.c: In function ‘main’:
    newuidmap.c:107:40: warning: comparison of integer expressions of different signedness: ‘int’ and ‘long unsigned int’ [-Wsign-compare]
      107 |         if ((written <= 0) || (written >= sizeof(proc_dir_name))) {
          |                                        ^~