Improve logging

[nirs]

This does not fix all the issues mentioned in #43 but prepare the infrastructure. With these changes adding timestamps for all logs is a trivial change.

• Log everything to stderr - this has many advantages: much easier to
  understand what happened before errors, much easier to understand the
  daemon state, since stderr is line buffered, so events (e.g. start,
  stop, connect, disconnect) are logged immediately. Easier to get logs
  from users.
• Replace fprintf and printf with logging macros (ERRORF, ERROR, INFO)
• Replace perror with ERRORN, showing the same output an error log.
• Use prefix for all logs ("DEBUG|", "INFO |", "ERROR|") to make the
  meaning of the message clear, and make all log message
  aligned nicely.
• Remove unneeded log for vmnet_return_t when we get VMNET_SUCCESS
• Replace print_vmnet_status() with vmnet_strerror() returning readable
  error name. Callers use ERRORF() to log the vmnet function name,
  return value and error name.
• Log socket length when it is too long
• Log the signal name and number
• Don't use perror() for logging errors from function snot setting errno
• Don't log same error twice (once in a function, second in the caller)

Example log with this change

# tail -f /var/log/socket_vmnet/stderr 
INFO | Initializing vmnet.framework (mode 1001)
INFO | * vmnet_subnet_mask: 255.255.255.0
INFO | * vmnet_mtu: 1500
INFO | * vmnet_end_address: 192.168.105.254
INFO | * vmnet_start_address: 192.168.105.1
INFO | * vmnet_interface_id: 3924D245-6997-472D-A263-97E2A27FF173
INFO | * vmnet_max_packet_size: 1514
INFO | * vmnet_nat66_prefix: fd9b:5a14:ba57:e3d3::
INFO | * vmnet_mac_address: 6a:d6:97:42:f0:20
INFO | Accepted a connection (fd 5)
INFO | Accepted a connection (fd 6)
INFO | Accepted a connection (fd 7)
INFO | Connection closed by peer (fd 7)
INFO | Closing a connection (fd 7)
INFO | Connection closed by peer (fd 6)
INFO | Closing a connection (fd 6)
INFO | Connection closed by peer (fd 5)
INFO | Closing a connection (fd 5)

[nirs]

This should move to log.h so we can split the to code to multiple files and use consistent logging.

[AkihiroSuda]

I guess we should use https://developer.apple.com/documentation/os/os_log ?

[AkihiroSuda]

See man 3 os_log for the usage

[nirs]

Sure, I'll try this.

But I think we should keep the logging macros so it is easy to change the logging backend later.

[AkihiroSuda]

Do you plan to update this?

[nirs]

What I have here was good enough for testing and fixing various issue, and I did not have time to check this direction so far.

I can update with with the version I run locally on top of current master, and we can add an issue to consider os_log.

We anyway need to add timestamps, using os_log will avoid this work.

[nirs]

Added an issue to consider using os_log: #64

[AkihiroSuda]

Thanks

[nirs]

Opened #70 to update docs/launchd configuration.