Use uv_set_vterm_state() to override libuv's guess.
See https://github.com/libuv/libuv/pull/1873/ for discussion.

This commit uses a terminal-detection approach based on
GetProcessImageFileNameW(...), which will be reverted in the following
commit. The approach was intended to handle the case of running in
winpty (:terminal), but we will add $NVIM env var for that.

Also add some support for ConEmu, cygwin.

Headers were moved in v1.21.0
https://github.com/libuv/libuv/commit/99ae3edf28fd5c848e886e41ac3f4bd40538e24d

Remove libuv-overlapped.patch since UV_OVERLAPPED_PIPE was included in
libuv v1.21.0:
https://github.com/libuv/libuv/commit/62a0f763a7d1a5bbaa2670b9d2687ba183d2c528

Notable changes since v1.12:
- 1.16.0
  - uv_os_getppid(): get parent PID
  - "win,tty: improve SIGWINCH support" (v1.15.0)
- 1.18.0
  - uv_os_getpid()
- 1.19.0
  - Windows: uv_kill() pid 0 now means "current process group", like unix.
    https://github.com/libuv/libuv/commit/890eedaf59cea75faaa6a14b4248a472dcadb831
- 1.20.0
  - unix,spawn: respect user stdio flags for new pipe
    https://github.com/libuv/libuv/commit/c409b3fcff51ea270724e4f43866764e65666095

otherwise, system headers cannot be found on Mojave
they were moved from /usr/include to SDK path

prevent luajit to assigning default value to MACOSX_DEPLOYMENT_TARGET
ref #9050

In the 20180331 release, the format was slightly changed:

> 20180331
>         + improve terminfo write/read by modifying the fourth item of the
>           extended header to denote the number of valid strings in the extended
>           string table (prompted by a comment in unibilium's sources).

Since the number of valid string capabilities is not necessarily the
same as extstrslen, it's not possible to sanity check the total number
of items up front anymore.