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

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

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

- Use pget(PGET_CANDEBUG | PGET_NOTWEXIT) to determine if the specified
PID is valid for monitoring in FILEMON_SET_PID ioctl.

- Set the monitored PID to -1 when the process exits.

Suggested by: jilles
Tested by: sjg
MFC after: 3 days

Match the options of the kernel.

Iterate rather than use recursion. We can blow out the kernel stack if there
is a long chain of fork(2)s.

There's no need to make filemon specific to i386 and amd64. All
LP64 architectures define elf64_freebsd_sysvec and all ILP32
architectures define elf32_freebsd_sysvec.

Add the 'filemon' device. 'filemon' is a kernel module that provides a device
interface for processes to record system calls of its children.

Submitted by: Juniper Networks.