Provide vectored variants of smgrread() and smgrwrite().

smgrreadv() and smgrwritev() and their md.c implementations call
FileReadV() and FileWriteV().  A range of disk blocks beginning at
'blocknum' and extending for 'nblocks' can be scattered to or gathered
from multiple buffers with a single system call.  The traditional
smgrread() and smgrwrite() functions are implemented in terms of the new
functions.

Later commits will introduce calls with nblocks > 1, but the following
behavioral changes can be seen already:

* After a short transfer we'll now retry until we eventually read 0
  bytes (= EOF) or get ENOSPC, EDQUOT, EFBIG etc, where previously we
  would infer the reason.  Retrying is consistent with xlog.c's
  treatment of large WAL writes, and arguably also xlog.c and fd.c's
  treatment of EINTR.  Arbitrary short returns for larger transfers have
  been observed on several OSes, and might in theory also happen for
  transient reasons with our own pg_p*v() fallback code.

* After unexpected EOF or -1, the error thrown now talks about
  a range even for the single block case, eg "blocks 42..42".

Reviewed-by: Heikki Linnakangas <hlinnaka@iki.fi>
Discussion: https://postgr.es/m/CA+hUKGJkOiOCa+mag4BF+zHo7qo=o9CFheB8=g6uT5TUm2gkvA@mail.gmail.com

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