use readdir() instead of the deprecated readir_r()

use readdir() instead of the deprecated readir_r()

readir() is threadsafe on both linux and freebsd anyway.
The rationale behind the readdir_r() deprecation is in the glibc manual:
https://www.gnu.org/software/libc/manual/html_node/Reading_002fClosing-Directory.html

This fixes the following warning with recent glibc:

linux.c: In function ‘list_ports’:
linux.c:197:2: warning: ‘readdir_r’ is deprecated [-Wdeprecated-declarations]
  while (!readdir_r(dir, &entry, &result) && result) {
  ^~~~~