Copy stable/10@r272459 to releng/10.1 as part of
the 10.1-RELEASE process.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

MFC r260581: Add sglist_append_bio(9) to append a struct bio's data to
a sglist

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Change the 'resid' parameter to sglist_consume_uio() from an int to a
size_t to match the recent type change of the uio_resid member of struct
uio.

Approved by: re (kib)

Add a simple API to manage scatter/gather lists of phyiscal addresses.
Each list describes a logical memory object that is backed by one or more
physical address ranges. To minimize locking, the sglist objects
themselves are immutable once they are shared.

These objects may be used in the future to facilitate I/O requests using
physically-addressed buffers. For the immediate future I plan to use them
to implement a new type of VM object and pager.

Reviewed by: jeff, scottl
MFC after: 1 month