Improve compatibility with 16-color terminals.

Fish assumed that you could use tparm setaf/bg as long
as the color was under 16 without even checking terminfo.

Now, we will always let tparm take care of a color if it
can. It will never use tparm if the terminal indicates a lack
of support. When tparm cannot safely take care of this, we
construct the escape ourselves. Now, fish no longer makes
such a assumption, either, that it can use a fancy 256-color
format for colors 0-16 when tparm can't do it natively.
Add an intermediate formatter that can handle colors 0-16.

That fixes the issue when tparm was pushed beyond its limits:

env TERM='linux' ./fish -c 'set_color --background brred | xxd'

... and producing bogus escape output.

Restores harmony to white, black ANSI color names and removes "grey".

Fixes fish-shell#3176

src/color.cpp

@@ -161,8 +161,8 @@ static const named_color_t named_colors[] = {
     {L"magenta", 5, {0xFF, 0, 0xFF}},
     {L"purple", 5, {0xFF, 0, 0xFF}},
     {L"cyan", 6, {0, 0xFF, 0xFF}},
-    {L"grey", 7, {0xE5, 0xE5, 0xE5}},
-    {L"brgrey", 8, {0x55, 0x55, 0x55}},
+    {L"white", 7, {0xE5, 0xE5, 0xE5}},
+    {L"brblack", 8, {0x55, 0x55, 0x55}},
     {L"brred", 9, {0xFF, 0x55, 0x55}},
     {L"brgreen", 10, {0x55, 0xFF, 0x55}},
     {L"brbrown", 11, {0xFF, 0xFF, 0x55}},
@@ -171,7 +171,7 @@ static const named_color_t named_colors[] = {
     {L"brmagenta", 13, {0xFF, 0x55, 0xFF}},
     {L"brpurple", 13, {0xFF, 0x55, 0xFF}},
     {L"brcyan", 14, {0x55, 0xFF, 0xFF}},
-    {L"white", 15, {0xFF, 0xFF, 0xFF}},
+    {L"brwhite", 15, {0xFF, 0xFF, 0xFF}},
 };
 
 wcstring_list_t rgb_color_t::named_color_names(void) {

src/output.cpp

@@ -44,8 +44,8 @@ void output_set_writer(int (*writer)(char)) {
 
 int (*output_get_writer())(char) { return out; }
 
-// Returns true if we think the term256 support is "native" as opposed to forced.
-static bool term256_support_is_native(void) { return max_colors >= 256; }
+/// Returns true if we think tparm can handle outputting a color index 
+static bool term_supports_color_natively(unsigned int c) { return max_colors >= c; }
 
 color_support_t output_get_color_support(void) { return color_support; }
 
@@ -59,15 +59,22 @@ unsigned char index_for_color(rgb_color_t c) {
 }
 
 static bool write_color_escape(char *todo, unsigned char idx, bool is_fg) {
-    bool result = false;
-    if (idx < 16 || term256_support_is_native()) {
+    if (term_supports_color_natively(idx)) {
         // Use tparm to emit color escape.
         writembs(tparm(todo, idx));
-        result = true;
+        return true;
     } else {
         // We are attempting to bypass the term here. Generate the ANSI escape sequence ourself.
-        char buff[128] = "";
-        snprintf(buff, sizeof buff, "\x1b[%d;5;%dm", is_fg ? 38 : 48, idx);
+        char buff[64] = "";
+        if (idx < 16) {
+            // 30-37 select foreground color, 40-47 background. \e%d for dim, \e%d;1 for bright
+            snprintf(buff, sizeof buff, "\x1b[%d%sm",
+                     is_fg ? 30 + ((idx > 8) ? idx - 8 : idx) :
+                             40 + ((idx > 8) ? idx - 8 : idx),
+                     idx < 8 ? "" : ";1");
+        } else {
+            snprintf(buff, sizeof buff, "\x1b[%d;5;%dm", is_fg ? 38 : 48, idx);
+        }
 
         int (*writer)(char) = output_get_writer();
         if (writer) {
@@ -76,9 +83,8 @@ static bool write_color_escape(char *todo, unsigned char idx, bool is_fg) {
             }
         }
 
-        result = true;
+        return true;
     }
-    return result;
 }
 
 static bool write_foreground_color(unsigned char idx) {