patch 8.2.0863: cannot set a separate color for underline/undercurl

Problem:    Cannot set a separate color for underline/undercurl.
Solution:   Add the t_AU and t_8u termcap codes. (Timur Celik, closes #6011)
diff --git a/src/term.h b/src/term.h
index dca48d1..bff901e 100644
--- a/src/term.h
+++ b/src/term.h
@@ -78,6 +78,7 @@
     KS_MB,	// blink mode
     KS_CAF,	// set foreground color (ANSI)
     KS_CAB,	// set background color (ANSI)
+    KS_CAU,	// set underline color (ANSI)
     KS_LE,	// cursor left (mostly backspace)
     KS_ND,	// cursor right
     KS_CIS,	// set icon text start
@@ -100,6 +101,7 @@
     KS_U7,	// request cursor position
     KS_8F,	// set foreground color (RGB)
     KS_8B,	// set background color (RGB)
+    KS_8U,	// set underline color (RGB)
     KS_CBE,	// enable bracketed paste mode
     KS_CBD,	// disable bracketed paste mode
     KS_CPS,	// start of bracketed paste
@@ -179,6 +181,7 @@
 #define T_MB	(TERM_STR(KS_MB))	// blink mode
 #define T_CAF	(TERM_STR(KS_CAF))	// set foreground color (ANSI)
 #define T_CAB	(TERM_STR(KS_CAB))	// set background color (ANSI)
+#define T_CAU	(TERM_STR(KS_CAU))	// set underline color (ANSI)
 #define T_LE	(TERM_STR(KS_LE))	// cursor left
 #define T_ND	(TERM_STR(KS_ND))	// cursor right
 #define T_CIS	(TERM_STR(KS_CIS))	// set icon text start
@@ -200,6 +203,7 @@
 #define T_U7	(TERM_STR(KS_U7))	// request cursor position
 #define T_8F	(TERM_STR(KS_8F))	// set foreground color (RGB)
 #define T_8B	(TERM_STR(KS_8B))	// set background color (RGB)
+#define T_8U	(TERM_STR(KS_8U))	// set underline color (RGB)
 #define T_BE	(TERM_STR(KS_CBE))	// enable bracketed paste mode
 #define T_BD	(TERM_STR(KS_CBD))	// disable bracketed paste mode
 #define T_PS	(TERM_STR(KS_CPS))	// start of bracketed paste