Remove dead code and clean up formatting

[HunterZ]

There's a bunch of dead code in the codebase:

• Commented / #ifdef'd out stuff
• Porting hacks/variations for platforms I no longer want to support
• Unused macros/methods/variables?

Also need to get down to one common implementation that supports Windows, Mac, and Linux, with no/minimal macrobatics. This means leaning on BearLibTerminal for I/O as much as possible, and trying to stick to ANSI C as much as possible for everything else (file I/O etc.).

[HunterZ]

Formatting and style guide:

Basic text formatting:

• no tabs anywhere, only spaces
• 2 spaces per indent level
• try to line wrap around 76-ish characters

Basic code formatting:

• use only slash-star /*...*/ comment style
• curly braces on their own lines as a general rule
• try to align stuff for readability where practical

Flow control formatting/structure:

• always use curly brace blocks under while/if/etc. if they contain multiple lines of indented code under them (original codebase would nest a loop inside another loop, and not use braces under the outer loop - no!)
• if a while/if/etc. contains only one line, prefer putting them on the same line if they'll fit together
• use curly brace blocks with a one-line else following a multi-line if and vice versa, etc.
• check the positive condition first in an if...else, to avoid the else being a double-negative
• avoid unnecessary else blocks around code that follows an if block ending in a break/continue/return statement
• use break/continue/return where possible to avoid unnecessary else blocks
• goto is long out of vogue, so try to avoid/eliminate it

Loop/increment/decrement semantics:

• prefer counting up to counting down, especially when zero is one of the boundaries (as this may only be safe with signed types)
• prefer pre-increment/decrement (++i) to post-increment/decrement (i--)

Pointer/array semantics:

• prefer array semantics to pointer increment/decrement for readability

Code structure:

• prefer reusable functions to code duplication
• prefer checking sad-path up-front and bailing out early versus nesting happy-path ifs
• define local variables at the beginning of functions
• provide one blank line between local variable definitions and the start of a function's actual logic

Variables/types:

• setting default variable values is encouraged (maybe want to revisit this as a cleanup item), but nontrivial values should be set where they're used versus up-front
• avoid spaces between casts and what they apply to
• (revisit? not my preferred style) pointer type designator asterisk * goes next to the variable name, not the type (exception: *const because there's no alternative)

Functions:

• (revisit if pedantic compiler warnings appear?) don't use (void) to swallow return values - it makes the code overly verbose
• use const arguments where possible

Macros:

• prefer #ifdef to #if defined() when checking a single macro condition
• prefer #if...#elif...#else...#endif to nesting when possible

[HunterZ]

Regarding ANSI C:

• Update function signatures from K&R to ANSI style, including header prototypes.
• Try to move away from POSIX-heavy stuff and towards ANSI C stuff as much as possible, as Windows support of POSIX API is dicey at best.
• Can't really get away from access() for now, but it's pretty compatible and simple on Windows.
• It's kind of silly, but I want to avoid C99, C++, etc. for now (maybe on a later pass through the codebase, during or after Restructure the codebase #3).