Support for RGB colour, using the extended cell mechanism to avoid

wasting unnecessary space. The 'Tc' flag must be set in the external
TERM entry (using terminal-overrides or a custom terminfo entry), if not
tmux will map to the closest of the 256 or 16 colour palettes.

Mostly from Suraj N Kurapati, based on a diff originally by someone else.

tmux.1

@@ -2921,7 +2921,7 @@ and poor for interactive programs such as shells.
 .Op Ic on | off
 .Xc
 Allow programs to change the window name using a terminal escape
-sequence (\\033k...\\033\\\\).
+sequence (\eek...\ee\e\e).
 The default is on.
 .Pp
 .It Xo Ic alternate-screen
@@ -4024,7 +4024,7 @@ This command only works from outside
 .El
 .Sh TERMINFO EXTENSIONS
 .Nm
-understands some extensions to
+understands some unofficial extensions to
 .Xr terminfo 5 :
 .Bl -tag -width Ds
 .It Em Cs , Cr
@@ -4048,10 +4048,12 @@ $ printf '\e033[4 q'
 If
 .Em Se
 is not set, \&Ss with argument 0 will be used to reset the cursor style instead.
+.It Em \&Tc
+Indicate that the terminal supports the
+.Ql direct colour
+RGB escape sequence (for example, \ee[38;2;255;255;255m).
 .It Em \&Ms
-This sequence can be used by
-.Nm
-to store the current buffer in the host terminal's selection (clipboard).
+Store the current buffer in the host terminal's selection (clipboard).
 See the
 .Em set-clipboard
 option above and the