Properly handle move up zero lines in TerminalMenus.

[GunnarFarneback]

Cc @timholy

Unfortunately #38489 wasn't quite correct in the case of a one element menu and at least one header line, as can be seen by running

using REPL.TerminalMenus
request(MultiSelectMenu(["1"]))

This was quite mysterious until I realized that the current implementation relies on the corner case that "\x1b[0A" ("move up zero lines") actually moves up one line. In fact the 0 parameter means to use the default value, which for cursor up is one, see https://vt100.net/docs/vt510-rm/chapter4.html, section 4.3.3.

This PR fixes moving up zero lines by conditionally doing nothing and removes a workaround related to this corner case. As an effect a one element menu is now drawn without a dummy empty line below it. This is not a regression from Julia 1.5, where a one element menu was an error.

Additionally I replaced the "\x1b[999D" control code ("move left 999 spaces") with a mundane carriage return. As far as I can tell that's the desired result.

[timholy]

"\x1b[0A" ("move up zero lines") actually moves up one line. In fact the 0 parameter means to use the default value, which for cursor up is one, see https://vt100.net/docs/vt510-rm/chapter4.html, section 4.3.3.

Wow, talk about a non-generic API. Julia has spoiled me.

I replaced the "\x1b[999D" control code ("move left 999 spaces") with a mundane carriage return

That seems safe since now we know that there is at least one line to clear. I imagine the former interface was intending to clear the current line.