realpath: let --relative-to default to --relative-base

realpath: let --relative-to default to --relative-base

Most of the time, if someone wants to filter which paths are
relative while leaving all others absolute, they also want to
to the filtering based on the same --relative-to directory.
Make this easier to specify.

* src/realpath.c (main): Convert error to default.
* doc/coreutils.texi (realpath invocation): Document this.
* tests/misc/realpath: Adjust test to match.
* NEWS: Document it.