Seems like redundant env var separators (";" on Windows) in $PATH can
cause weird behavior. From #7377:

> After some time, system(['win32yank', '-o']) and system('win32yank -o')
> start returning different results: specifically first returns an
> empty string.
>
> 1. $PATH weirdly contains double semicolon followed by path to the
>    “installation directory” (unpacked directory from archive).
> 2. If I run `let $PATH=substitute($PATH, ';;', ';', 'g')` the problem is fixed.

close #7377
ref 224f99b85d311ebd31451db13b66e4a3c7e51938

close #11459

Initially done in 28e59cb22, but does not appear to be necessary
anymore.

Uses UV_EEXIST directly, just like UV_ENOENT.

- Minimum required libuv is now v1.12
- Because `uv_os_getenv` requires allocating, we must manage a map
  (`envmap` in `env.c`) to maintain the old behavior of `os_getenv` .
- free() map-items after removal. khash.h does not make copies of
  anything, so even its keys must be memory-managed by the caller.

closes #8398
closes #9267

ref https://github.com/neovim/neovim/issues/9001#issuecomment-421843790

Steps to reproduce:
    :set verbose=9
    :call system(['echo'])
    E730: using List as a String

libuv users are only supposed to directly include uv.h.  In v1.21.0, all
the uv-*.h headers were renamed to uv/*.h, which caused the unit tests
to fail with

    [123/125] Generating post/uv-errno.h
    FAILED: test/includes/post/uv-errno.h
    cd «SRCDIR»/src/neovim/build/test/includes && /usr/bin/clang -std=c99 -E -P «SRCDIR»/src/neovim/test/includes/pre/uv-errno.h -I/usr/include -I/usr/include -o «SRCDIR»/neovim/build/test/includes/post/uv-errno.h
    «SRCDIR»/src/neovim/test/includes/pre/uv-errno.h:1:10: error: 'uv-errno.h' file not found with <angled> include; use "quotes" instead
    #include <uv-errno.h>
             ^~~~~~~~~~~~
             "uv-errno.h"

The intention of the file is to extend libuv's error constants with more
values used by the unit tests.  This can just as easily be achieved
without poking into pseudo-private header files.