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

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

/* -> /*- for license, minor formatting changes

Remove advertising clause from University of California Regent's license,
per letter dated July 22, 1999.

Approved by: core

Move the typedef for size_t into _iovec.h, so that size_t is available
for struct iovec.

It is bad style to define the same structure in multiple header
files which might be included together.

Things like debuggers and lint-like programs get their knickers in
a twist (rightly so one might add) when they find different locations
for the same named struct depending on which .h file were included
first.

This is a stellar example of Very Bad Thinking on the part of the
standards dudes who wrote that both sys/uio.h and sys/socket.h
should define struct iovec the same way.

Fix this by putting struct iovec into its own miniature sys/_iovec.h
file and #include that from sys/socket.h and sys/uio.h.

Sensible people could just put iovec into sys/_types.h but there
is probably some standard or other which will be violated if we
did something that horrible.