Don't accidentially generate trigraphs in filewrapper

[MackieLoeffel]

When trying out this library, I ran into the problem of the sandboxee crashing with the following error on optimized builds (debug builds worked fine):

[comms.cc : 561] RAW: read: Connection reset by peer [104]
[forkserver.cc : 96] RAW: Receiving init PID from the ForkServer failed
WARNING: Logging before InitGoogleLogging() is written to STDERR
E0325 13:18:53.440409 13504 executor.cc:157] Could not obtain init PID
[comms.cc : 535] RAW: write: Bad file descriptor [9]
[forkserver.cc : 68] RAW: Sending PB to the ForkServer failed

I traced to problem down to a SIGSEGV, when spawning the embedded child and the problem turned out to be, that parts of the embedded data were interpreted as trigraphs by gcc. This PR fixes this problem by emitting \? instead of ? in the filewrapper.

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[MackieLoeffel]

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