patch 8.0.1722: cannot specify a minimal size for a terminal window

Problem:    Cannot specify a minimal size for a terminal window.
Solution:   Support the "rows*cols" format for 'winsize'.

runtime/doc/options.txt

@@ -7960,15 +7960,23 @@ A jump table for the options with a short description can be found at |Q_op|.
 'termsize' 'tms'	string	(default "")
 			local to window
 			{not in Vi}
-	Size of the |terminal| window.  Format: {rows}x{columns}.
+	Size of the |terminal| window.  Format: {rows}x{columns} or
+	{rows}*{columns}.
 	- When empty the terminal gets the size from the window.
-	- When set (e.g., "24x80") the terminal size is not adjusted to the
-	  window size.  If the window is smaller only the top-left part is
-	  displayed.
-	  When rows is zero then use the height of the window.
-	  When columns is zero then use the width of the window.
-	  For example: "30x0" uses 30 rows with the current window width.
-	  Using "0x0" is the same as empty.
+	- When set with a "x" (e.g., "24x80") the terminal size is not
+	  adjusted to the window size.  If the window is smaller only the
+	  top-left part is displayed.
+	- When set with a "*" (e.g., "10*50") the terminal size follows the
+	  window size, but will not be smaller than the specified rows and/or
+	  columns.
+	- When rows is zero then use the height of the window.
+	- When columns is zero then use the width of the window.
+	- Using "0x0" or "0*0" is the same as empty.
+
+	Examples:
+	  "30x0" uses 30 rows and the current window width.
+	  "20*0" uses at least 20 rows and the current window width.
+	  "0*40" uses the current window height and at least 40 columns.
 	Note that the command running in the terminal window may still change
 	the size of the terminal.  In that case the Vim window will be
 	adjusted to that size, if possible.