Prevent eagerCache=false from causing undefined behavior

[Witiko]

In a486b88, we documented that disabling the eagerCache Lua option can lead to undefined behavior when markdown fragments are nested. However, this is not always true: disabling the eagerCache Lua option only causes undefined behavior when \markdownMode is set to the legacy values 0 (shell escape via \write18) and 1 (shell escape via os.execute()), not 2 (direct Lua access). Furthermore, we can detect nested markdown fragments and avoid undefined behavior: If we are in a nested context and \markdownMode is 2, then we disable eagerCache and redirect output from \markdownOptionOutputTempFileName to a new type of a temporary output file.

[Witiko]

Perhaps a simpler solution that leverages the semantics of files on unix: Currently, we open outputTempFileName for writing, which truncates the file, affecting all file descriptors open for reading. However, if we first unlinked the file from outputTempFileName and then opened outputTempFileName for writing, a new file would be created and the original should be garbage-collected after the end of the process, making any conflict between the two different files impossible.

[Witiko]

This issue will be solved when Witiko/lt3luabridge#8 has been closed and after we have removed \markdownMode in #309. Then, we can remove the undefined behavior notice and make eagerCache=false the default behavior.