lowpkg: partially unify public functions signature

In the same way that the previous commit for pkg, add **kwargs
into the public function signature for lowpkg virtual module, to
unify the different signatures.

salt/modules/dpkg_lowpkg.py

@@ -135,7 +135,7 @@ def unpurge(*packages):
     return salt.utils.data.compare_dicts(old, new)
 
 
-def list_pkgs(*packages):
+def list_pkgs(*packages, **kwargs):
     '''
     List the packages currently installed in a dict::
 
@@ -169,7 +169,7 @@ def list_pkgs(*packages):
     return pkgs
 
 
-def file_list(*packages):
+def file_list(*packages, **kwargs):
     '''
     List the files that belong to a package. Not specifying any packages will
     return a list of _every_ file on the system's package database (not
@@ -211,7 +211,7 @@ def file_list(*packages):
     return {'errors': errors, 'files': list(ret)}
 
 
-def file_dict(*packages):
+def file_dict(*packages, **kwargs):
     '''
     List the files that belong to a package, grouped by package. Not
     specifying any packages will return a list of _every_ file on the system's