[lldb] [llgs server] Support creating core dumps on NetBSD

Add a new SaveCore() process method that can be used to request a core
dump.  This is currently implemented on NetBSD via the PT_DUMPCORE
ptrace(2) request, and enabled via 'savecore' extension.

Protocol-wise, a new qSaveCore packet is introduced.  It accepts zero
or more semicolon-separated key:value options, invokes the core dump
and returns a key:value response.  Currently the only option supported
is "path-hint", and the return value contains the "path" actually used.
The support for the feature is exposed via qSaveCore qSupported feature.

Differential Revision: https://reviews.llvm.org/D101285

lldb/source/Utility/StringExtractorGDBRemote.cpp

@@ -260,6 +260,8 @@ StringExtractorGDBRemote::GetServerPacketType() const {
       break;
 
     case 'S':
+      if (PACKET_STARTS_WITH("qSaveCore"))
+        return eServerPacketType_qLLDBSaveCore;
       if (PACKET_STARTS_WITH("qSpeedTest:"))
         return eServerPacketType_qSpeedTest;
       if (PACKET_MATCHES("qShlibInfoAddr"))