Add a "--reverse" or "-r" option to the "list" with no options command.  This will list backwards from the
last source point listed.
Also fix the setting of the default file & line to the file containing main, when you do a plain "list".

<rdar://problem/12685226>


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@171945 91177308-0d34-0410-b5e6-96231b3b80d8