Added a boolean to the pure virtual lldb_private::Process::CanDebug(...)

method so process plug-ins that are requested by name can answer yes when
asked if they can debug a target that might not have any file in the target.

Modified the ConnectionFileDescriptor to have both a read and a write file
descriptor. This allows us to support UDP, and eventually will allow us to
support pipes. The ConnectionFileDescriptor class also has a file descriptor
type for each of the read and write file decriptors so we can use the correct
read/recv/recvfrom call when reading, or write/send/sendto for writing.

Finished up an initial implementation of UDP where you can use the "udp://"
URL to specify a host and port to connect to:

(lldb) process connect --plugin kdp-remote udp://host:41139

This will cause a ConnectionFileDescriptor to be created that can send UDP
packets to "host:41139", and it will also bind to a localhost port that can
be given out to receive the connectionless UDP reply. 

Added the ability to get to the IPv4/IPv6 socket port number from a 
ConnectionFileDescriptor instance if either file descriptor is a socket.

The ProcessKDP can now successfully connect to a remote kernel and detach
using the above "processs connect" command!!! So far we have the following
packets working:
    KDP_CONNECT
    KDP_DISCONNECT
    KDP_HOSTINFO
    KDP_VERSION
    KDP_REATTACH

Now that the packets are working, adding new packets will go very quickly.

llvm-svn: 135363