Problem:  ":lang messages en_US.UTF-8" no longer overrides the language
          detected from the environment (at startup).
Solution: In os_setenv, special-case "LC_ALL", "LANG", et al. to use
          putenv_s instead of uv_os_setenv.

fixes #11045

Without this "sysinit uses VIM if XDG_CONFIG_DIRS unset" fails with an
existing `/etc/xdg/nvim/sysinit.vim` file, because `XDG_CONFIG_DIRS=` is
not considered, but the default ("/etc/xdg") used then.

Pulse every 0.1s only.

This makes `!yes` look much better (less busy).

Problem:    May free memory twice when out of memory.
Solution:   Check that backslash_halve_save() returns a different pointer.
            (Dominique Pelle, closes vim/vim#4847)
https://github.com/vim/vim/commit/f1552d07d715b437d941659479942c2543b02bd4

  ../src/nvim/os/fs.c: In function 'os_can_exe':
  ../src/nvim/os/fs.c:247:27: warning: passing argument 1 of 'is_executable_ext' discards 'const' qualifier from pointer target type [-Wdiscarded-qualifiers]
    247 |     if (is_executable_ext(name, abspath)) {
        |                           ^~~~
  In file included from ../src/nvim/os/fs.c:36:
  src/nvim/auto/os/fs.c.generated.h:7:38: note: expected 'char *' but argument is of type 'const char *'
      7 | static _Bool is_executable_ext(char *name, char **abspath) FUNC_ATTR_NONNULL_ARG(1);
        |                                ~~~~~~^~~~
  ../src/nvim/os/fs.c: In function 'os_resolve_shortcut':
  ../src/nvim/os/fs.c:1183:56: warning: conversion from 'size_t' {aka 'const long long unsigned int'} to 'int' may change value [-Wconversion]
   1183 |     const int conversion_result = utf8_to_utf16(fname, len, &p);
        |                                                        ^~~
  ../src/nvim/os/fs.c:1211:19: warning: declaration of 'conversion_result' shadows a previous local [-Wshadow]
   1211 |         const int conversion_result = utf16_to_utf8(wsz, -1, &rfname);
        |                   ^~~~~~~~~~~~~~~~~
  ../src/nvim/os/fs.c:1183:15: note: shadowed declaration is here
   1183 |     const int conversion_result = utf8_to_utf16(fname, len, &p);
        |               ^~~~~~~~~~~~~~~~~

- take a size parameter
- return libuv error code
- handle error in caller only (avoid redundant messages)

https://github.com/libuv/libuv/commit/53995a3825d23eacd01e2bcfa35642c4a188d32b
https://github.com/libuv/libuv/commit/4c945f49365ab4d6e1b07bf0ef2893455dc04622