- Moved into util.cc
- Added forward_to to skip trace data to some arbitrary point in time

We need to be able to expose this data so it can
be serialized.

Digs out original executable image that this task was forked
from, or in the case of exec, exec'd on.

This is required for persistent checkpointing, so that the names in the
proc fs corresponds to a correct name at replay time (i.e. has the same
behavior/looks the same in proc fs as a normal replay). The thread name is
not what should be showing up in /proc/tid/comm, but the actual
executable. So we need to be able to find this "original exe" of the
task.

Required for the create checkpoints command, etc. to determine what
events in the trace are checkpointable, when not having a live session.

In future commits/PRs, remove the static function in ReplaySession.cc`
that does the same thing and use this member function on Event instead.

Gets additional proc fs paths for a task, in this case
/mem. Required for persistent checkpointing to figure out
on how to handle mappings and what to serialize (and what not to
serialize).

The function extract_name will also be required for setting up syscall
buffer stuff in coming commits.

Need to be able to set this data when restoring an address space.

Added persistent checkpoint schema for capnproto rr_pcp.capnp,
as well a compile command for it in CMakeLists.txt, that works like
the other one (rr_trace.capnp)

CheckpointInfo and MarkData types works as intermediaries between a
serialized checkpoint and a deserialized "live" one. MarkData is used for
copying the contents of Mark, InternalMark, ProtoMark and it's various
data into, for serialization as well when deserializing, to reconstruct
those types.

The reasoning for adding MarkData is to not intrude in Mark/InternalMark/ProtoMark
interface and possibly break some guarantees or invariants they provide.
If something goes wrong now, it's constrained only to persistent
checkpointing not reconstituting a session properly.

GDB spawned by RR now has 2 additional commands, write-checkpoints, which
serializes any checkpoints set by the `checkpoint` command and
load-checkpoints.

Added the rr create-checkpoints command which create persistent checkpoints
on a specified interval, which it attempts to honor as closely as possible.

RerunCommand and ReplayCommand are now aware of PCPs.

Replay sessions get spawned from persistent checkpoints if they
exist on disk when using `-g <evt>` or when using `-f <pid>` and that
"task" was created some time after a persistent checkpoint.

Added the --ignore-pcp flag to these commands, which ignores pcps
and spawns sessions normally.

Restored comments, that existed in static function in ReplaySession.cc
Change all use of this to Event::can_checkpoint_at
Removed static can_checkpoint_at in ReplaySession.cc

Since checkpoints are partially initialized, checking that they are is pointless.

Deserializing and serializing an FdTable is now performed by the class itself instead of in a free function

FileMonitor has a public member function that is used for serialization.
Each derived type that requires special/additional logic, extends
the virtual member function serialize_type.

not necessary for serialization, as FdTable is separately restored.

Task::copy_state sets the OS name of a task in the same fashion that
persistent checkpointing sets name. Refactored this functionality into
Task::set_name.

Also removed the unnecessary `update_prname` from Task::copy_state.

update_prname is not a "write to tracee"-operation but a "read from tracee"-operation; and since
we already know what name we want to set Task::prname to, we skip this reading from the tracee
in Task::copy_state and just set it to the parameter passed in to Task::set_name

Refactor so that marks_with_checkpoints is just changed in one place, not arbitrarily access it. Ref counts had the same changes in a previous commit.

Fixes a bug for loaded persistent checkpoints where the re-created checkpoints did not get their reference counting correct.

This closes rr-debugger#3678