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

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

Copy head to stable/9 as part of 9.0-RELEASE release cycle.

Approved by: re (implicit)

Clean up compilation warnings. Set WARNS=6 in Makefile.

PR: 71659
Submitted by: Dan Lukes <dan obluda cz>

Move common credential save and restore code into a separate file.
Improve credential handling in pt_file.c

Add a new pipe sub-namespace.
This allows us to:
- perform scatter gather operations without using temporary files,
- create non-linear pipelines, and
- implement file views using symbolic links.

File view idea by: Vassilios Karakoidas
Portalfs pointer by: John Ioannidis
MFC after: 1 month