Relax absolut path checking in our 'file' scheme implementation

Relax absolut path checking in our 'file' scheme implementation

So far, we strictly obeyed [RFC 8089], which only allows absolute paths
in a 'file:' URI.  However, this seems to give a confusing user
experience, where something like 'file:foo.pem' wouldn't open foo.pem,
even though it's there in the current directory, but 'file:$(pwd)/foo.pem'
would.

To be less surprising for such use cases, we relax our implementation
visavi [RFC 8089] to allow relative paths.

[RFC 8089]: https://datatracker.ietf.org/doc/html/rfc8089

Fixes #27461

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/27482)