Copy stable/9 to releng/9.3 as part of the 9.3-RELEASE cycle.

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

MFC 246120,246148,246206,246587,247411,247415:
Add fmemopen(3), open_memstream(3), and open_wmemstream(3) which provide
stdio FILE objects for memory buffers.

port exprun by: bdrewery

Add an implementation of open_memstream() and open_wmemstream(). These
routines provide write-only stdio FILE objects that store their data in a
dynamically allocated buffer. They are a string builder interface somewhat
akin to a completely dynamic sbuf.

Reviewed by: bde, jilles (earlier versions)
MFC after: 1 month