unistd.h and libc, and add a man page.
Allow wiggle room in the man page for implementations of fallocate
that either (a) don't fully unwind on failure, leaving new blocks
allocated without changing the file size, or (b) create only in-memory
transient reservations that disappear when crashing or rebooting.
Also, add crossreference to fdiscard from ftruncate(2), and remove the
old BUGS entry from there that called for a generalized version that
allows discarding ranges of a file: that's what fdiscard is.
