Re-add databases/p5-DBD-Oracle removed with r435735 as there is user demand for it.
Tested by Konstantin Stroikovsky (maintainer of databases/oracle8-client).
Additional changes:

• take maintainership;
• simplify port: remove traces of oracle7-client and options OCI7/OCI8;
• bump PORTREVISION because of these changes.

Approved by: vsevolod (mentor)