vserver plugin: Use `readdir_r' instead of `readdir'.

vserver plugin: Use `readdir_r' instead of `readdir'.

`readdir' is not thread safe, so use the (more thread safe) `readdir_r'.
Also, don't use the non-standard `d_type' member of the `dirent'
structure - it's not portable.

I'm aware that this plugin is very Linux specific and Linux most likely
will never use another C library than the GNU libc, but using this
member prevents me from compiling with the most restrictive `strict'
settings..