There are at least three problems with using git-annex
on `/sdcard` on Android, or on a FAT filesystem, or on (to a first
approximation) Windows:

1. symlinks
2. hard links
3. unix permissions

So, I've added an `annex.crippledfilesystem` setting. `git annex init` now
probes to see if all three are supported, and if not, enables that, as well
as direct mode.

In crippled filesystem mode, all the permissions settings are skipped.
Most of them are only used to lock down content in the annex in indirect
mode anyway, so no great loss.

There are several uses of hard links, most of which can be dealt with by
making copies. The one use of permissions and hard links I absolutely
needed to deal with was that they're used to lock down a file as it's being
ingested into the annex. That can't be done on crippled filesystems, so I
made it instead check the metadata of the file before and after to detect
if it changed, the same way direct mode detects when files are modified.
This is likely better than the old method anyway.

The other reason files are hardlinked while they're being ingested is that
this allows running lsof on a single directory of files that are in the
process of being added, to detect if anything has them open for write.
I still need to adjust the lsof check to work in crippled filesystem mode.
It seems this won't make it much slower to run lsof on the whole repository.

At this point, I can use git-annex with a repository on `/sdcard` or a FAT
filesystem, and at least `git annex add` works. 

Still several things on the TODO list before crippled filesystem mode is
complete. The only one I'm scared about is making `git merge` do something
sane when it wants to merge two trees full of symlinks, and the filesystem
doesn't let it create a symlink..
