Problem:    Various warnings from clang on MS-Windows.
Solution:   Avoid the warnings. (Yegappan Lakshmanan, closes vim/vim#10553)

https://github.com/vim/vim/commit/a34b4460c2843c67a35a2d236b01e6cb9bc38734

Co-authored-by: Yegappan Lakshmanan <yegappan@yahoo.com>

On exit, pty_process_close() may be called after pty_process_finish1()
but before start_wait_eof_timer(), in which case the timer shouldn't be
started because pty_process_close() has already closed it.

This should prevent use-after-free on exit on Windows.

Then we can just load metadata in C as a single msgpack blob. Which also
can be used directly as binarly data, instead of first unpacking all the
functions and ui_events metadata to immediately pack it again, which was
a bit of a silly walk (and one extra usecase of `msgpack_rpc_from_object`
which will get yak shaved in the next PR)

refactor(fileio): remove API shell layer encouraging unnecessary allocations

Functions like file_open_new() and file_open_fd_new() which just is a
wrapper around the real functions but with an extra xmalloc/xfree around
is an anti-pattern. If the caller really needs to allocate a
FileDescriptor as a heap object, it can do that directly.

FileDescriptor by itself is pretty much a pointer, or rather two:
the OS fd index and a pointer to a buffer. So most of the time an extra
pointer layer is just wasteful.

In the case of scriptin[curscript] in getchar.c, curscript used
to mean in practice:

N+1 open scripts           when curscript>0
zero or one open scripts   when curscript==0

Which means scriptin[0] had to be compared to NULL to disambiguate the
curscript=0 case.

Instead, use curscript==-1 to mean that are no script,
then all pointer comparisons dissappear and we can just use an array of
structs without extra pointers.

os_getenv("FOO") caches the result when $FOO is set to something
non-empty. However, when $FOO was not set, every new call to
os_getenv("FOO") would allocate a temporary scratch buffer to
immediately throw away.

This has an huge impact e.g. on logging which depends on potentially
non-set env vars.

In the context a String inside an Object/Dictionary etc is consumed,
it is considered to be read-only.