Use the process' ReadCStringFromMemory from DYLDRendezvous::ReadStringFromMemory.

Tested:
Ubuntu 14.04 x86_64, clang-3.5-built lldb.
MacOSX 10.9.4, Xcode6-Beta7-built lldb.

Visual packet inspection on lldb <-> llgs shows significant reduction in overly-verbose
memory read traffic on start-up when scanning shared library info.

Change by Stephane Sezer.