Kernel+LibC: Add a dump_backtrace() syscall.

This is very simple but already very useful. Now you're able to call to
dump_backtrace() from anywhere userspace to get a nice symbolicated
backtrace in the debugger output. :^)

Kernel/Process.cpp

@@ -2718,3 +2718,9 @@ int Process::sys$mknod(const char* pathname, mode_t mode, dev_t dev)
 
     return VFS::the().mknod(StringView(pathname), mode, dev, current_directory());
 }
+
+int Process::sys$dump_backtrace()
+{
+    dump_backtrace();
+    return 0;
+}

Kernel/Process.h

@@ -104,6 +104,7 @@ public:
     void die();
     void finalize();
 
+    int sys$dump_backtrace();
     int sys$gettid();
     int sys$donate(int tid);
     int sys$shm_open(const char* name, int flags, mode_t);

Kernel/Syscall.cpp

@@ -291,6 +291,8 @@ static u32 handle(RegisterDump& regs, u32 function, u32 arg1, u32 arg2, u32 arg3
     case Syscall::SC_reboot: {
         return current->process().sys$reboot();
     }
+    case Syscall::SC_dump_backtrace:
+        return current->process().sys$dump_backtrace();
     default:
         kprintf("<%u> int0x82: Unknown function %u requested {%x, %x, %x}\n", current->process().pid(), function, arg1, arg2, arg3);
         return -ENOSYS;

Kernel/Syscall.h

@@ -115,7 +115,8 @@ struct timeval;
     __ENUMERATE_SYSCALL(sched_getparam)         \
     __ENUMERATE_SYSCALL(fchown)                 \
     __ENUMERATE_SYSCALL(halt)                   \
-    __ENUMERATE_SYSCALL(reboot)
+    __ENUMERATE_SYSCALL(reboot)                 \
+    __ENUMERATE_SYSCALL(dump_backtrace)
 
 namespace Syscall {
 

Libraries/LibC/unistd.cpp

@@ -526,4 +526,10 @@ int reboot()
     int rc = syscall(SC_reboot);
     __RETURN_WITH_ERRNO(rc, rc, -1);
 }
+
+void dump_backtrace()
+{
+    syscall(SC_dump_backtrace);
+}
+
 }

Libraries/LibC/unistd.h

@@ -14,6 +14,7 @@ __BEGIN_DECLS
 
 extern char** environ;
 
+void dump_backtrace();
 int fsync(int fd);
 void sysbeep();
 int systrace(pid_t);