MFC r321839:

Implement proper Linux /dev/fd and /proc/self/fd behavior by adding
Linux specific things to the native fdescfs file system.

Unlike FreeBSD, the Linux fdescfs is a directory containing a symbolic
links to the actual files, which the process has open.
A readlink(2) call on this file returns a full path in case of regular file
or a string in a special format (type:[inode], anon_inode:<file-type>, etc..).
As well as in a FreeBSD, opening the file in the Linux fdescfs directory is
equivalent to duplicating the corresponding file descriptor.

Here we have mutually exclusive requirements:

• in case of readlink(2) call fdescfs lookup() method should return VLNK

vnode otherwise our kern_readlink() fail with EINVAL error;

• in the other calls fdescfs lookup() method should return non VLNK vnode.

For what new vnode v_flag VV_READLINK was added, which is set if fdescfs has beed
mounted with linrdlnk option an modified kern_readlinkat() to properly handle it.

For now For Linux ABI compatibility mount fdescfs volume with linrdlnk option:

mount -t fdescfs -o linrdlnk null /compat/linux/dev/fd

Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D11452