Fix edge cases and consolidate implementations of file I/O

[Rangi42]

RGBDS comprises four programs, all of which have multiple kinds of input and/or output. There are a number of issues with how they implement that:

• Implementations are scattered, often duplicating functionality (such as handling of - for stdin/stdout).
• read()/fread() might read fewer bytes than you requested, e.g. if the file is a pipe. That's not an error, it just means you need to loop more calls until finished.
• Unlimited-length pseudo-device files like /dev/zero and /dev/urandom can behave oddly. For example, INCBIN "/dev/zero" hangs indefinitely.
• Potential TOCTTOU race conditions between finding a definite file path among the -Include directories with fstk_FindFile, and actually opening the file.

We would probably benefit from more high-level C++ STL file I/O, since we're manually doing things with read(), fread(), std::filebuf, etc, when std::ifstream exists.

[Rangi42]

These are theoretical issues that haven't caused actual problems for users -- nobody is really doing INCBIN "/dev/zero", or modifying a file in the split second between when INCLUDE "file" is parsed and when it's executed, or so on. And refactoring+consolidating file I/O would risk introducing new bugs (our test suite isn't enough, real-world use on lots of different machines would be necessary -- we don't even test a lot of the fatal-error edge-cases for things like fopen failure.) So I'm okay with postponing this til 2.0.