posixfs: also accept relative paths, see #23

URLS now might look like:
- file:///absolute/path
- file://relative/path (*)

Internally, PosixFS now:
- accepts relative paths as path argument
- always works with an absolute .base_path internally

(*) not sure if that is the final solution, consider
this as a temporary hack.

src/borgstore/backends/posixfs.py

@@ -15,10 +15,10 @@
 
 
 def get_file_backend(url):
-    # file:///var/backups/borgstore/first
+    # file:///absolute/path or file://relative/path
     file_regex = r"""
         file://
-        (?P<path>(/.*))
+        (?P<path>(.*))
     """
     m = re.match(file_regex, url, re.VERBOSE)
     if m:
@@ -27,7 +27,7 @@ def get_file_backend(url):
 
 class PosixFS(BackendBase):
     def __init__(self, path, *, do_fsync=False):
-        self.base_path = Path(path)
+        self.base_path = Path(path).absolute()
         self.opened = False
         self.do_fsync = do_fsync  # False = 26x faster, see #10