wasip2: Don't reimplement preopens.c (#655)

This is really tricky path-related code and it's currently buggy on
wasip2. Instead of duplicating it like before instead have only a single
definition with some small handling of wasip1-vs-wasip2. This fixes
`chdir`, for example, and handling of preopens. The fundamental bug from
before is that a `wasip2_string_t` was handed out as a `const char*`,
but it wasn't null-terminated. The bug is more apparent with all the
code paths interleaved here.

This additionally fixes an issue where the previous implementation would
continuously open new file descriptors for preopens. Instead now a
single file descriptor is registered at preopens-registration-time which
is then used throughout the lifetime of the program.

Author: alexcrichton

expected/wasm32-wasip2/defined-symbols.txt

@@ -339,7 +339,6 @@ __wasilibc_open_nomode
 __wasilibc_populate_preopens
 __wasilibc_pthread_self
 __wasilibc_random
-__wasilibc_register_preopened_fd
 __wasilibc_rename_newat
 __wasilibc_rename_oldat
 __wasilibc_reset_preopens

libc-bottom-half/headers/public/wasi/libc.h

@@ -21,11 +21,8 @@ struct timespec;
 /// afterward, you should call this before doing so.
 void __wasilibc_populate_preopens(void);
 
+#ifndef __wasilibc_use_wasip2
 /// Register the given pre-opened file descriptor under the given path.
-#ifdef __wasilibc_use_wasip2
-int __wasilibc_register_preopened_fd(filesystem_preopens_own_descriptor_t fd,
-                                     wasip2_string_t relprefix);
-#else
 int __wasilibc_register_preopened_fd(int fd,
                                      const char* prefix);
 #endif