libc: add helper furnction to set sysctl() user.* variables

Testing had revealed that trying to retrieve the user.localbase
variable into to small a buffer would return the correct error code,
but would not fill the available buffer space with a partial result.

A partial result is of no use, but this is still a violation of the
documented behavior, which has been fixed in the previous commit to
this function.

I just checked the code for "user.cs_path" and found that it had the
same issue.

Instead of fixing the logic for each user.* sysctl string variable
individually, this commit adds a helper function set_user_str() that
implements the semantics specified in the sysctl() man page.

It is currently only used for "user.cs_path" and "user.localbase",
but it will offer a significant simplification when further such
variables will be added (as I intend to do).

MFC after: 3 days