patch 8.2.0518: a terminal falls back to setting $TERM to "xterm"

Problem:    A terminal falls back to setting $TERM to "xterm".
Solution:   Use "xterm-color" if more than 16 colors are supported and
            "xterm-256color" if at least 256 colors are supported.
            (closes #5887)

src/os_unix.c

@@ -5493,7 +5493,14 @@ mch_job_start(char **argv, job_T *job, jobopt_T *options, int is_terminal)
 	    // Use 'term' or $TERM if it starts with "xterm", otherwise fall
 	    // back to "xterm".
 	    if (term == NULL || *term == NUL || STRNCMP(term, "xterm", 5) != 0)
-		term = "xterm";
+	    {
+		if (t_colors > 16)
+		    term = "xterm-color";
+		if (t_colors >= 256)
+		    term = "xterm-256color";
+		else
+		    term = "xterm";
+	    }
 	    set_child_environment(
 		    (long)options->jo_term_rows,
 		    (long)options->jo_term_cols,

src/version.c

@@ -738,6 +738,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    518,
 /**/
     517,
 /**/