Allow absolute paths to be set

Allow absolute paths to be set

It was a mistake to allow relative paths for include files (just
like root shouldn't have "." in its PATH), but we probably can't
change it now. Add a new pragma "abspath" that someone can put
in the system-wide config file to require absolute paths.

Also update the config documentation to better explain how file
inclusion works.

Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15090)