patch 8.0.1609: shell commands in the GUI use a dumb terminal

Problem:    Shell commands in the GUI use a dumb terminal.
Solution:   Add the "!" flag to 'guioptions' to execute system commands in a
            special terminal window.  Only for Unix now.
diff --git a/src/option.h b/src/option.h
index 09c044e..d2ee545 100644
--- a/src/option.h
+++ b/src/option.h
@@ -214,6 +214,7 @@
 #define SHM_ALL		"rmfixlnwaWtToOsAIcqF" /* all possible flags for 'shm' */
 
 /* characters for p_go: */
+#define GO_TERMINAL	'!'		/* use terminal for system commands */
 #define GO_ASEL		'a'		/* autoselect */
 #define GO_ASELML	'A'		/* autoselect modeless selection */
 #define GO_BOT		'b'		/* use bottom scrollbar */
@@ -236,7 +237,7 @@
 #define GO_FOOTER	'F'		/* add footer */
 #define GO_VERTICAL	'v'		/* arrange dialog buttons vertically */
 #define GO_KEEPWINSIZE	'k'		/* keep GUI window size */
-#define GO_ALL		"aAbcefFghilmMprtTvk" /* all possible flags for 'go' */
+#define GO_ALL		"!aAbcefFghilmMprtTvk" /* all possible flags for 'go' */
 
 /* flags for 'comments' option */
 #define COM_NEST	'n'		/* comments strings nest */