Federal Charter Sought for S&L;: American Continental Corp. has filed an application with the Federal Home Loan Bank Board to convert a subsidiary, Lincoln Savings & Loan, from a state-chartered to a federally chartered institution, regulators said. Should the application be approved, Irvine-based Lincoln would no longer be regulated by the California Department of Savings and Loans. Any sale would then require the sole approval of the Federal Home Loan Bank Board in Washington. The bank board has 60 days to act on the request, a board spokesman. California regulators have already turned down one potential buyer for Lincoln. Neither American Continental executives nor bank board officials would say why the change was requested.