The intention is to replace usages of char* by uint8_t* in DetectExecutableType.
For this, the function ReadOrFile is replaced by a class
BufferedFileReader, which provide an accessor on the data.
This will allow us to easily investigate on using MemoryMappedFile.

BUG=

Review-Url: https://codereview.chromium.org/2140203002
Cr-Commit-Position: refs/heads/master@{#405333}