Make build hermetic without shell scripts

- Fix some minor issues in ar.com
- Have execve() look for `ape` command
- Rewrite NT paths using /c/ rather /??/c:/
- Replace broken GCC symlinks with .sym files
- Rewrite $PATH environment variables on startup
- Make $(APE_NO_MODIFY_SELF) the default bootloader
- Add all build command dependencies to build/bootstrap
- Get the repository mostly building from source on non-Linux

tool/build/runitd.c

@@ -305,6 +305,7 @@ void Recv(void *output, size_t outputsize) {
     // pass along eof condition to zlib
     INFOF("mbedtls_ssl_read");
     received = mbedtls_ssl_read(&ezssl, buf, sizeof(buf));
+    if (!received) TlsDie("got unexpected eof", received);
     if (received < 0) TlsDie("read failed", received);
     // decompress packet completely
     // into a dynamical size buffer