Implement true color support define in:
https://en.wikipedia.org/wiki/ANSI_escape_code

The sequence is:
ESC[ … 38;2;;; … m Select RGB foreground color
ESC[ … 48;2;;; … m Select RGB background color

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

This helps catch Perl syntax/version/dependency issues at build time
rather than run time.

A couple of them got improved out of existence.

(revert assert(false) added by 4835dcf5)
(fixes #918)

These cause warnings from static checkers, and seem to lead to spurious
GCC7 -Wmaybe-uninitialized.

There are many more.

Some of these are large and contain smaller unrelated refactors.

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

This slows terminal emulation slightly.