improve mountpoint parsing

improve mountpoint parsing

changes from v1:
renamed function to verify_*
added check for ../ at the beginning
cleaned up regex (\.)? -> \.?

currently we sanitize mountpoints with sanitize_mountpoint, which
tries to remove dots, double-dots and multiple slashes, but it does it
not correctly (e.g. /test/././ gets truncated to /test./ )

instead of trying to truncate the path, we create a format for mp strings
which throws an error if /./ or /../ exist (also /. and /.. at the end or
../ at the beginning) since there should be no valid use for these in
mountpoint paths anyway

with the new behaviour, we don't need sanitize_mountpoint anymore:

Signed-off-by: Dominik Csapak <d.csapak@proxmox.com>