Call restore_original_signal_state after GDB forks.

When I run GDB under Valgrind, GDB seems to segfault
displaying:

Fatal signal: Segmentation fault
----- Backtrace -----
0x2803f7 ???
0x3c9696 ???
0x3c9899 ???
0x55f8fcf ???
0x486c000 ???
---------------------
A fatal error internal to GDB has been detected, further
debugging is not possible.  GDB will now terminate.

This is a bug, please report it.  For instructions, see:
<https://www.gnu.org/software/gdb/bugs/>.

warning: linux_ptrace_test_ret_to_nx: PC 0x5821c09d is neither near return address 0x486c000 nor is the return instruction 0x4f8f4a!

but then, acts like nothing happened and excutes normally. This is
because it's the child from linux_ptrace_test_ret_to_nx that
segfaults and parent GDB carries on normally. Restore the original
signal states to not to print confusing backtrace. After restoring,
only such warning is displayed:

warning: linux_ptrace_test_ret_to_nx: WSTOPSIG 19 is neither SIGTRAP nor SIGSEGV!

Author: sasshka

gdb/nat/linux-ptrace.c

@@ -23,6 +23,7 @@
 #include <sys/procfs.h>
 #endif
 #include "gdbsupport/eintr.h"
+#include "gdbsupport/signals-state-save-restore.h"
 
 /* Stores the ptrace options supported by the running kernel.
    A value of -1 means we did not check for features yet.  A value
@@ -148,6 +149,9 @@ linux_ptrace_test_ret_to_nx (void)
       return;
 
     case 0:
+      /* Set signal handlers to their default because it doesn't make sense
+	 to call GDB-specific handlers any more in the child process. */
+      restore_original_signals_state ();
       l = ptrace (PTRACE_TRACEME, 0, (PTRACE_TYPE_ARG3) NULL,
 		  (PTRACE_TYPE_ARG4) NULL);
       if (l != 0)