dbghelp: Don't parse the DWARF info from Mach-O files if we were requested to only provide public symbols.

This now works more like how the ELF and PE support works.

For backward compatibility with old, buggy GCC, Apple uses a different register
numbering scheme for the eh_frame section.

See, for reference, the comments near the top of this file from LLDB's source:
https://github.com/llvm-mirror/lldb/blob/release_36/source/Plugins/Process/Utility/RegisterContext_x86.h

From: Michael Müller <michael@fds-team.de>

Also lock/unlock the module only on creation/destruction of the object.

Also lock/unlock the module only on creation/destruction of the object.