FreeBSD: acl_get: raise OSError if lpathconf fails

Previously:
- acl_get just returned for lpathconf returning EINVAL
- acl_get silently ignored all other lpathconf errors and
  implied it is not a NFS4 acl

Now:
- not sure why the EINVAL silent return was done, but it seems
  wrong. guess it could be the system not implementing a check
  for nfs4. but in that case guess we still would like to get
  the default and access ACL!? Thus, I removed the silent return.
- raise OSError for all lpathconf errors

Cosmetic: add a nfs4_acl boolean, so the code reads better.

src/borg/platform/freebsd.pyx

@@ -145,6 +145,7 @@ def acl_get(path, item, st, numeric_ids=False, fd=None):
     If `numeric_ids` is True the user/group field is not preserved only uid/gid
     """
     cdef int flags = ACL_TEXT_APPEND_ID
+    flags |= ACL_TEXT_NUMERIC_IDS if numeric_ids else 0
     if isinstance(path, str):
         path = os.fsencode(path)
     ret = acl_extended_link_np(path)
@@ -154,10 +155,10 @@ def acl_get(path, item, st, numeric_ids=False, fd=None):
         # there is no ACL defining permissions other than those defined by the traditional file permission bits.
         return
     ret = lpathconf(path, _PC_ACL_NFS4)
-    if ret < 0 and errno.errno == errno.EINVAL:
-        return
-    flags |= ACL_TEXT_NUMERIC_IDS if numeric_ids else 0
-    if ret > 0:
+    if ret < 0:
+        raise OSError(errno.errno, os.strerror(errno.errno), os.fsdecode(path))
+    nfs4_acl = ret == 1
+    if nfs4_acl:
         _get_acl(path, ACL_TYPE_NFS4, item, 'acl_nfs4', flags, fd=fd)
     else:
         _get_acl(path, ACL_TYPE_ACCESS, item, 'acl_access', flags, fd=fd)