Implement handling of `library:` keys in thread stop replies.

[sas]

Summary

When a windows remote stops because of a DLL load/unload, the debug server
sends a stop reply packet that contains a library key with any value (usually
just library:1). This indicates to the debugger that a library has been
loaded or unloaded and that the list of libraries should be refreshed (usually
with qXfer:libraries:read).

This change just triggers a call to LoadModules() which in turns will send a
remote library read command when a stop reply that requests it is received.

[sas]

Fix bug with bool comparison.

[tberghammer]

I leave this review to the others as I have no idea about how library loading works on Windows. (On Linux/Android we use a breakpoint on a special symbol exposed by the linker)

[clayborg]

Looks good.

source/Plugins/Process/gdb-remote/ProcessGDBRemote.cpp

case 'S':
                     if (wp_sp)
                         wp_index = wp_sp->GetHardwareIndex();
 
                     reason = "watchpoint";
                     StreamString ostr;
                     ostr.Printf("%" PRIu64 " %" PRIu32, wp_addr, wp_index);
                     description = ostr.GetString().c_str();
                 }
+                else if (key.compare("library"))
+                {
+                    LoadModules();
+                }
                 else if (key.size() == 2 && ::isxdigit(key[0]) && ::isxdigit(key[1]))
                 {
                     uint32_t reg = StringConvert::ToUInt32 (key.c_str(), UINT32_MAX, 16);
                     if (reg != UINT32_MAX)
                         expedited_register_map[reg] = std::move(value);
                 }
             }