Various updates to the ftruncate(2) documentation:

• Note that ftruncate(2) can operate on shared memory objects and cross reference shm_open(2).
• Note that ftruncate(2) does not change the file position pointer (aka seek pointer) of the file descriptor.
• ftruncate(2) will fail with EINVAL for all sorts of other fd types than just sockets, so instead note that it fails for all but regular files and shared memory objects.
• Note that ftruncate(2) also appeared in 4.2BSD along with truncate(2). (Or at least the manpage for both appeared in 4.2, I did not check the kernel code itself to see if either predated 4.2.)

PR: 199472 (2)
Submitted by: andrew@ugh.net.au (2)
MFC after: 1 week