Implement `posix_getdents`

[mkroening]

We currently implement the following non-standardized Linux interface for getting directory entries, getdents(2):

ssize_t getdents64(int fd, void dirp[.count], size_t count);

struct linux_dirent64 {
    ino64_t        d_ino;    /* 64-bit inode number */
    off64_t        d_off;    /* Not an offset; see getdents() */
    unsigned short d_reclen; /* Size of this dirent */
    unsigned char  d_type;   /* File type */
    char           d_name[]; /* Filename (null-terminated) */
};

d_off is a filesystem-specific value with no specific meaning to user space, though on older filesystems it used to be the distance from the start of the directory to the start of the next linux_dirent; see readdir(3).

We could gradually adopt the following standardized interface, posix_getdents, <dirent.h>:

ssize_t posix_getdents(int fildes, void *buf, size_t nbyte, int flags);

struct posix_dent {
    ino_t          d_ino      File serial number.
    reclen_t       d_reclen   Length of this entry, including trailing
                              padding if necessary. See posix_getdents().
    unsigned char  d_type     File type or unknown-file-type indication.
    char           d_name[]   Filename string of this entry.
};

This would mainly benefit from being standardized and something we can commit to. Additionally, it is nice to get rid of the very unusual d_off.