Made FILE an interface to various backends. This allows application writers
to override the standard FILE API functions with their own backends. This
is highly unportable - it'd be nice if a real standard existed for this.
glibc already does something like this internally, but AFAIK you can't hook
into it.

Added fdopen(3), fopen(3), fregister(3), funregister(3), fread(3),
fwrite(3), fseek(3), clearerr(3), ferror(3), feof(3), rewind(3), ftell(3),
fflush(3), fclose(3), fileno(3), fnewline(3), fcloseall(3), memset(3),
stdio(3), vfprintf(3), fprintf(3), and vprintf(3).

Added a file-descriptor backend to the FILE API.

fd's {0, 1, 2} are now initialized as stdin, stdout, and stderr when the
standard library initializes.

fcloseall(3) is now called on exit(3).

decl/intn_t_.h now @include(size_t.h) instead of declaring it itself.

Added <stdint.h>.

The following programs now flush stdout: cat(1), clear(1), editor(1),
init(1), mxsh(1).

printf(3) is now hooked up against vprintf(3), while Maxsi::PrintF
remains using the system call, for now.