Use pclose() instead of fclose()

Use pclose() instead of fclose()

The return value from popen() is a normal standard I/O stream in all
respects save that it must be closed with pclose() rather than
fclose(3).

Solves the following warning when compiled with -Wmismatched-dealloc:

errno.c: In function ‘search_all’:
errno.c:126:5: warning: ‘fclose’ called on pointer returned from a
                         mismatched allocation function
                         [-Wmismatched-dealloc]
  126 |     fclose(f);
      |     ^~~~~~~~~
errno.c:113:9: note: returned from ‘popen’
  113 |     f = popen("locale -a", "r");
      |         ^~~~~~~~~~~~~~~~~~~~~~~