True color escape sequences are even more complicated

It's recently been discovered that everybody's been implementing true-color escape sequences with colons wrong, it seems.

People have been implementing ESC [38:2:0:255:0m, but it should apparently be ESC [38:2:<colorspace>:0:255:0m, according to https://bugzilla.gnome.org/show_bug.cgi?id=791456 and https://gitlab.com/gnachman/iterm2/issues/6377. libvte and iTerm2 have both added support for this new, correct variant.

Currently this is an academic point because Mosh only implements the semicolon variant, and I don't think anybody has any desire to change that variant's behavior for compatibility reasons. But when we do parse the colon variant, we should probably handle the old, mistaken variant as well as this new variant.

Thanks to George Nachman's ITerm2 release notes for pointing this out and catching my eye.

Start by locating Mosh's escape-sequence parser and its existing semicolon true-color handling. Read the cited GNOME VTE and iTerm2 references, then determine the intended handling for both colon variants; done means the parser accepts the correct colorspace form and preserves compatibility with the older form.