Convert one or both of a string representation of a UID and GID into 
integer form.  On platforms where 
pwd and 
grp is 
available, user and group names can be converted.
| Parameters | uid | A string giving the base-ten representation of a UID or the name of a user 
which can be converted to a UID via pwd.getpwnam, or None if 
no UID value is to be obtained.
 (type: str or NoneType
) | 
 | gid |  (type: str or NoneType
) | 
 | uid | A string giving the base-ten representation of a GID or the name of a group
which can be converted to a GID via grp.getgrnam, or None if 
no UID value is to be obtained.
 | 
| Returns | A two-tuple giving integer UID and GID information for whichever (or both) 
parameter is provided with a non-None value.
 | 
| Raises | ValueError | If a user or group name is supplied and pwd or 
grp is not available.
 |