fss(4): Allow FSSIOCSET to set the initial flags. Add a new flag
"FSS_UNLINK_ON_CREATE" to unlink the backing store before
the snapshot gets created.

With this change dump(8) no longer dumps the zero-sized, but named
snapshot it is working on. Same applies to fsck_ffs(8).

branches: 1.14.2;
File system snapshots are no longer experimental.

branches: 1.13.2;
Use .In instead of .Aq Pa for header files.

branches: 1.12.2;
Obsolete fssbs(4). The kernel thread is named fssN now.

Kernel thread changed name from fssbsN to fssN.

branches: 1.10.2;
Convert TNF licenses to new 2 clause variant

branches: 1.9.20; 1.9.22;
Drop trailing whitespace.

Support user controllable flags for the snapshot driver:
- Add FSSIOFSET and FSSIOFGET ioctl() to set/get the flags.
- Add FSS_UNCONFIG_ON_CLOSE flag to unconfigure the snapshot device
on the last close.

Reviewed by: Jason R. Thorpe <thorpej@netbsd.org>

branches: 1.7.2;
Make page look better. Requested by uwe.

The fss command appeared -> The fss device appeared

branches: 1.5.2;
Add fssbs as Nm (for makewhatis/apropos).

Update the file system snapshot driver:
- Document the kernel thread.
- Rename some functions and variables.
- Return EROFS where appropriate.
- Use shifts instead of 64-bit divide.
- Use a simple_lock to make it MP-safe.
- Add M_CANFAIL to malloc to avoid panic on large cluster size.
- Allow sparse file for backing store and use VOP_BALLOC() to allocate
space. Default size of backing store is the size of the file system.

Fix Xref; from Jeff Rizzo in PR 23807.

Some nitfixes.

The file system snapshot pseudo driver.

Uses a hook in spec_strategy() to save data written from a mounted
file system to its block device and a hook in dounmount().

Not enabled by default in any kernel config.

Approved by: Frank van der Linden <fvdl@netbsd.org>