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Add test coverage for BOM-sniffing and zero byte files

Fix read_char_utf16le to correctly restore the position when mapping CRLF -> LF fails.

Refactoring: move seeking logic for "r+" and "a" modes from open_file to open_helper
open_helper is a better place for this logic because it already has special logic for each OPEN* mode.
This also eliminates some unnecessary seeking.

Merge in changes from trunk (tests for OPENUPDATE in binary mode)

Add test for using OPENAPPEND in binary mode.

Update OPENAPPEND to work correctly with Unicode content.

Merge in fix for Bug #536.
The fix will be modified for Unicode refactoring, but the tests are worth keeping.

Fix Bug #536; Make OPENAPPEND immediately set WRITEPOS to end of file.

Add FILETEST.OPENUPDATE.FILEEXITS, which was lost in a bad merge.

Merge changes from trunk to get minor updates to file.lgo