mfile_encoding: Add to officially documented functions. * libinterp/corefcn/input.cc (Fmfile_encoding): Rename from "F__mfile_encoding__". Add documentation. Add alias with the former name. * doc/interpreter/func.txi: Add documentation for function to manual. * doc/interpreter/basics.txi, libgui/src/main-window.cc (main_window::update_default_encoding), scripts/pkg/private/install.m (extract_pkg), scripts/testfun/__run_test_suite__.m, test/file-encoding/file-encoding.tst: Adapt for changed function name.

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