basic/terminal-util: introduce SYSTEMD_COLORS environment variable

... to determine if color output should be enabled. If the variable is not set,
fall back to using on_tty(). Also, rewrite existing code to use
colors_enabled() where appropriate.

src/basic/terminal-util.c

@@ -1135,3 +1135,16 @@ int open_terminal_in_namespace(pid_t pid, const char *name, int mode) {
 
         return receive_one_fd(pair[0], 0);
 }
+
+bool colors_enabled(void) {
+        const char *colors;
+
+        colors = getenv("SYSTEMD_COLORS");
+        if (!colors) {
+                if (streq_ptr(getenv("TERM"), "dumb"))
+                        return false;
+                return on_tty();
+        }
+
+        return parse_boolean(colors) != 0;
+}

src/basic/terminal-util.h

@@ -79,37 +79,38 @@ unsigned lines(void);
 void columns_lines_cache_reset(int _unused_ signum);
 
 bool on_tty(void);
+bool colors_enabled(void);
 
 static inline const char *ansi_underline(void) {
-        return on_tty() ? ANSI_UNDERLINE : "";
+        return colors_enabled() ? ANSI_UNDERLINE : "";
 }
 
 static inline const char *ansi_highlight(void) {
-        return on_tty() ? ANSI_HIGHLIGHT : "";
+        return colors_enabled() ? ANSI_HIGHLIGHT : "";
 }
 
 static inline const char *ansi_highlight_underline(void) {
-        return on_tty() ? ANSI_HIGHLIGHT_UNDERLINE : "";
+        return colors_enabled() ? ANSI_HIGHLIGHT_UNDERLINE : "";
 }
 
 static inline const char *ansi_highlight_red(void) {
-        return on_tty() ? ANSI_HIGHLIGHT_RED : "";
+        return colors_enabled() ? ANSI_HIGHLIGHT_RED : "";
 }
 
 static inline const char *ansi_highlight_green(void) {
-        return on_tty() ? ANSI_HIGHLIGHT_GREEN : "";
+        return colors_enabled() ? ANSI_HIGHLIGHT_GREEN : "";
 }
 
 static inline const char *ansi_highlight_yellow(void) {
-        return on_tty() ? ANSI_HIGHLIGHT_YELLOW : "";
+        return colors_enabled() ? ANSI_HIGHLIGHT_YELLOW : "";
 }
 
 static inline const char *ansi_highlight_blue(void) {
-        return on_tty() ? ANSI_HIGHLIGHT_BLUE : "";
+        return colors_enabled() ? ANSI_HIGHLIGHT_BLUE : "";
 }
 
 static inline const char *ansi_normal(void) {
-        return on_tty() ? ANSI_NORMAL : "";
+        return colors_enabled() ? ANSI_NORMAL : "";
 }
 
 int get_ctty_devnr(pid_t pid, dev_t *d);