* proc/core: off-by-one error reading ELF core files

core.(*splicedMemory).ReadMemory checked the entry interval
erroneously when dealing with contiguous entries.

* terminal,service,proc/*: adds dump command (gcore equivalent)

Adds the `dump` command that creates a core file from the target process.

Backends will need to implement a new, optional, method `MemoryMap` that
returns a list of mapped memory regions.
Additionally the method `DumpProcessNotes` can be implemented to write out
to the core file notes describing the target process and its threads. If
DumpProcessNotes is not implemented `proc.Dump` will write a description of
the process and its threads in a OS/arch-independent format (that only Delve
understands).

Currently only linux/amd64 implements `DumpProcessNotes`.

Core files are only written in ELF, there is no minidump or macho-o writers.

# Conflicts:
#	pkg/proc/proc_test.go