Add all FSMap constructor arguments as class attributes

[rabernat]

There are currently two constructor attributes for FSMap - create and check - that are not accessible as instance attributes. This makes it hard to copy an FSMap. Having these accessible would be useful for e.g. zarr-developers/zarr-python#911; it would allow us to completely rebuild an FSMap object from another.

[rabernat]

In looking at this code, the following scares me

filesystem_spec/fsspec/mapping.py

Lines 50 to 57 in 44f7b16

	if check:
	if not self.fs.exists(root):
	raise ValueError(
	"Path %s does not exist. Create "
	" with the ``create=True`` keyword" % root
	)
	self.fs.touch(root + "/a")
	self.fs.rm(root + "/a")

What if there is an array called /a in the store? Should this check path be sure to use a file that doesn't already exist?

[martindurant]

Good idea, a long unprobable string might be good. Do we really need check, I wonder.

[rabernat]

I have used check in the past to verify that the store is actually writeable. I believe that wrapping the touch in a try / except block and providing a very verbose error message would be even more useful.

[martindurant]

Fair enough

[martindurant]

The rest we can leave for a follow-up