Planned ABI break for 3.0

[klange]

With 3.0, I want to clean up some things which it has not been possible to change due to ABI compatibility.

Namely:

• Namespacing all of our library symbols. The graphics library is the most egregious case, where we have exported symbols like rgb and flip. Some others are just too generic, like (unfortunately) the hashmap library, which has conflicts in Netsurf. We can slap a toaru_ in front of nearly all of these if we need to.
• Changing the userspace entry calling convention. We've been implementing standard C calling conventions for program entry for a while, which is at least against the x86-64 SysV ABI that says we should just be passing argc, argv, etc. on the stack for _start.
• Removing the legacy stat and readdir interfaces (and restoring the unmangled names).
• Thread-local errno.
• Whatever else seems useful to do now before it becomes a problem later.

[klange]

pre_main also has to go

[klange]

Replacing environ with a weak alias to a new __environ is apparently also an ABI break, but only because we haven't been doing PIE - with PIE, references to environ will still compile to undefined references, but without PIE the linker creates a definition in the binary and the definition within the libc matters.