Some old platforms (NetBSD, AIX) implement the common stdio "putchar()"
function as a global macro which poisons the global namespace, preventing
all C and C++ programs from using "putchar()" as a function or method.
There was a long thread about this in fltk.coredev during the period
Mar 25 2024 ~ Apr 4 2024, subject "RFC: Fl_Terminal::putchar() in public API"
as to why we have no choice but to not use putchar() as a method name.
New public methods:
void clear(void);
void clear(Fl_Color val);
old protected methods made public:
void clear_screen(bool scroll_to_hist=true); // ESC [ 2 J
void clear_screen_home(bool scroll_to_hist=true); // ESC [ H ESC [ 2 J
void cursor_home(void); // ESC [ 0 H
test/terminal modified to test these, and added separate tests
for both the API and ANSI code ways to do these ops.
textcolor() needed for consistency, color() behavior documented.
Both added to the test/terminal app.
Elaborated on the special background "see through" color value + behavior,
tested with color() in test/terminal app.
