mkfs --subvol fixups

[kdave]

There are some fixups needed for the mkfs --subvol interface and some code cleanups. They should be separate patches on top of current code in devel (the hardlink code depends on that so we don't want to revert the changes). This issue tracks what needs to be done.

Syntax of --subvol:

mkfs --subvol SPEC

SPEC := [ TYPE ] path
TYPE := "default:" | "ro:" | "rw:" | "default-ro:"

Todo:

• docs: update according to the new syntax
• docs: mention that path that could match the type prefix can be worked around by ./
• update tests, cover all options, also with the hardlink support

Code style and API fixes:

• use path_ api for path manipulation, don't use dynamic allocation