Suppress SIGTTOU when handling errors

Calls to error () can cause SIGTTOU to send gdb to the background.

For example, on an Arm build:
  (gdb) b main
  Breakpoint 1 at 0x10774: file /build/gdb/testsuite/../../../src/binutils-gdb/gdb/testsuite/gdb.base/watchpoint.c, line 174.
  (gdb) r
  Starting program: /build/gdb/testsuite/outputs/gdb.base/watchpoint/watchpoint

  [1]+  Stopped                 ../gdb ./outputs/gdb.base/watchpoint/watchpoint
  localhost$ fg
  ../gdb ./outputs/gdb.base/watchpoint/watchpoint
  Cannot parse expression `.L1199 4@r4'.
  warning: Probes-based dynamic linker interface failed.
  Reverting to original interface.

The SIGTTOU is raised whilst inside a syscall during the call to tcdrain.
Fix is to use scoped_ignore_sigttou to ensure SIGTTOU is blocked.

In addition fix include comments - job_control is not included via terminal.h

gdb/ChangeLog:

	* event-top.c: Remove include comment.
	* inflow.c (class scoped_ignore_sigttou): Move from here...
	* inflow.h (class scoped_ignore_sigttou): ...to here.
	* ser-unix.c (hardwire_drain_output): Block SIGTTOU during drain.
	* top.c:  Remove include comment.

gdb/inflow.h

@@ -21,5 +21,36 @@
 #define INFLOW_H
 
 #include <unistd.h>
+#include <signal.h>
+#include "common/job-control.h"
+
+/* RAII class used to ignore SIGTTOU in a scope.  */
+
+class scoped_ignore_sigttou
+{
+public:
+  scoped_ignore_sigttou ()
+  {
+#ifdef SIGTTOU
+    if (job_control)
+      m_osigttou = signal (SIGTTOU, SIG_IGN);
+#endif
+  }
+
+  ~scoped_ignore_sigttou ()
+  {
+#ifdef SIGTTOU
+    if (job_control)
+      signal (SIGTTOU, m_osigttou);
+#endif
+  }
+
+  DISABLE_COPY_AND_ASSIGN (scoped_ignore_sigttou);
+
+private:
+#ifdef SIGTTOU
+  sighandler_t m_osigttou = NULL;
+#endif
+};
 
 #endif /* inflow.h */