patch 8.0.1665: when running a terminal from the GUI 'term' is not useful

Problem:    When running a terminal from the GUI 'term' is not useful.
Solution:   Use $TERM in the GUI if it starts with "xterm". (closes #2776)

runtime/doc/terminal.txt

@@ -352,7 +352,9 @@ On Unix a pty is used to make it possible to run all kinds of commands.  You
 can even run Vim in the terminal!  That's used for debugging, see below.
 
 Environment variables are used to pass information to the running job:
-    TERM		name of the terminal, from the 'term' option
+    TERM		the name of the terminal, from the 'term' option or
+			$TERM in the GUI; falls back to "xterm" if it does not
+			start with "xterm"
     ROWS		number of rows in the terminal initially
     LINES		same as ROWS
     COLUMNS		number of columns in the terminal initially

src/os_unix.c

@@ -5579,11 +5579,23 @@ mch_job_start(char **argv, job_T *job, jobopt_T *options)
 
 # ifdef FEAT_TERMINAL
 	if (options->jo_term_rows > 0)
+	{
+	    char *term = (char *)T_NAME;
+
+#ifdef FEAT_GUI
+	    if (term_is_gui(T_NAME))
+		/* In the GUI 'term' is not what we want, use $TERM. */
+		term = getenv("TERM");
+#endif
+	    /* 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";
 	    set_child_environment(
 		    (long)options->jo_term_rows,
 		    (long)options->jo_term_cols,
-		    STRNCMP(T_NAME, "xterm", 5) == 0
-						   ? (char *)T_NAME : "xterm");
+		    term);
+	}
 	else
 # endif
 	    set_default_child_environment();

src/version.c

@@ -762,6 +762,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1665,
 /**/
     1664,
 /**/