read_file_string quite inefficient and fails with binary files

[Nicolas]

The read_file_string function defined in lib/util.cpp is quite inefficient. It calls read_file_malloc to load the file into a malloc'd char buffer, then creates a std::string from that buffer and frees the buffer.

In addition, it doesn't pass the file length to the std::string constructor, so the string is cut at the first null byte in the file.

I attached two patches: one makes minor code cleanup to both read_file_string and read_file_malloc, the second rewrites read_file_string to do its own reading, without calling the _malloc version. (It's a bit of a mess; the basic operation without 'tail' and 'max_len' could be done in three lines if we were using C++ iostreams instead of FILE*, but whatever...)

(I haven't tested this patch beyond "it compiles"; I've been busy writing tests for and fixing a more serious bug)

[Nicolas]

The "data truncated at first null byte" issue causes #887.

[davea]

(In [17966]) - sample bitwise validator: make it work for binary files

fixes #886, #887

[Nicolas]

This wasn't really fixed, it was "wontfix"ed... (read_file_string is still inefficient, and still fails with binary, which is what I reported here)