Rename argc/argv main to __main_argc_argv (WebAssembly#134)

* Rename no-argument `main` to `__nullary_main`.

Change the ABI for the user entry point to rename `main` to
`__main_argc_argv` when it has argc/argv. This is needed because
wasm requires caller and callee signatures to match, so the
traditional trick of passing main arguments even if it doesn't
expect them doesn't work.

LLVM and related tools have been using the name `__original_main`
for a similar purpose, but that's a confusing name.

There's also a change here in that this is renaming the argc/argv
form rather than renaming the nullary form. The choice is somewhat
arbitrary, but I think it's slightly nicer to bias the aesthetics
toward the no-argument form, because that's the smaller and
simpler form.

* Fix typo.

* Reword and expand on the proposal.

Clarify the "entrypoint" terminology, and mention that export names
may differ from ABI symbol names.

BasicCABI.md

@@ -154,3 +154,46 @@ is no way to traverse the linear stack. There is also no currently-specified way
 is used as the frame pointer or base pointer. This functionality is not needed for backtracing or unwinding (since the
 wasm VM must do this in any case); however it may still be desirable to allow this functionality for debugging
 or in-field crash reporting. Future ABIs may designate a convention for determining frame size and local usage.
+
+## Program startup
+
+### User entrypoint
+
+The *user entrypoint* is the function which runs the bulk of the program.
+It is called `main` in C, C++, and other languages. Note that this may
+not be the first function in the program to be called, as programs may
+also have global constructors which run before it.
+
+At the wasm C ABI level, the following symbol names are used:
+
+C ABI Symbol name            | C and C++ signature                |
+---------------------------- | -----------------------------------|
+`main`                       | `int main(void)` or `int main()`   |
+`__main_argc_argv`           | `int main(int argc, char *argv[])` |
+
+These symbol names only apply at the ABI level; C and C++ source should
+continue to use the standard `main` name, and compilers will handle the
+details of conforming to the ABI.
+
+Also note that C symbol names are distinct from WebAssembly export
+names, which are outside the scope of the C ABI. Toolchains which export
+the user entrypoint may chose to export it as the name `main`, even when
+the C ABI symbol name is `__main_argc_argv`.
+
+A symbol name other than `main` is needed because the usual trick of
+having implementations pass arguments to `main` even when they aren't
+needed doesn't work in wasm, which requires caller and callee signatures
+to exactly match.
+
+For the same reason, the wasm C ABI doesn't support an `envp` parameter.
+Fortunately, `envp` is not required by C, POSIX, or any other relevant
+standards, and is generally considered obsolete in favor of `getenv`.
+
+### Program entrypoint
+
+The *program entrypoint* is the first function in the program to be called.
+It is commonly called `_start` on other platforms, though this is a
+low-level detail that most code doesn't interact with.
+
+The program entrypoint is out of scope for the wasm C ABI. It may depend
+on what environment the program will be run in.