Add support for resetting the terminal modes on stderr to something sensible

before printing error messages to it.  This should fix the stair-stepping
in Plink's progress messages.

[originally from svn r7745]

unix/unix.h

@@ -60,6 +60,18 @@ extern long tickcount_offset;
 #define WCHAR wchar_t
 #define BYTE unsigned char
 
+/*
+ * Unix-specific global flag
+ *
+ * FLAG_STDERR_TTY indicates that standard error might be a terminal and
+ * might get its configuration munged, so anything trying to output plain
+ * text (i.e. with newlines in it) will need to put it back into cooked
+ * mode first.  Applications setting this flag should also call
+ * stderr_tty_init() before messing with any terminal modes, and can call
+ * premsg() before outputting text to stderr and postmsg() afterwards.
+ */
+#define FLAG_STDERR_TTY 0x1000
+
 /* Things pty.c needs from pterm.c */
 char *get_x_display(void *frontend);
 int font_dimension(void *frontend, int which);/* 0 for width, 1 for height */
@@ -91,6 +103,12 @@ char *x_get_default(const char *key);
 /* Things uxstore.c provides to pterm.c */
 void provide_xrm_string(char *string);
 
+/* Things provided by uxcons.c */
+struct termios;
+void stderr_tty_init(void);
+void premsg(struct termios *);
+void postmsg(struct termios *);
+
 /* The interface used by uxsel.c */
 void uxsel_init(void);
 typedef int (*uxsel_callback_fn)(int fd, int event);