Use the cmake variable `WAMR_BUILD_GLOBAL_HEAP_POOL` and
`WAMR_BUILD_GLOBAL_HEAP_SIZE` to enable/disable the global heap pool
and set its size. And set the default global heap size in core/config.h and
the cmake files.
As a result, the developers who build iwasm can easily enable/disable the
global heap pool and change its size regardless of the iwasm implementation,
without manually finding and patching the right location for that value.
While compiling the file wasi_socket_ext.c with pedantic options (typically
`-Wimplicit-int-conversion` and `-Wmissing-prototypes`), some warnings are raised.
This PR addresses those warnings by adding missing static statements before
functions and explicitly casting a narrowing conversion.
And fix the error handling after calling getpeername.
