elnode should allow logging to processes and files

[nicferrier]

Elnode needs to be able to send errors and accesses to files and to processes instead of just buffers.

Elnode's current log handling is just buffers through elnode-log-access function and the elnode-error macro.

There tends to be 1 error log overall (accessed through the function elnode--get-error-log-buffer) and 1 access log per dispatcher instance (which is per application, normally) through elnode-hostpath-dispatcher.

Both deal with buffers only.

Processes

One should be able to specify a program name that could be invoked to handle log messages. Ideally it would be possible to maintain a connection to that program for a long period of time, to log many messages through it. If it fails it should automatically restart.

Files

There should be files backing up the log buffers. The implementation of these could be efficient through append-to-file and comint-truncate-buffer (which might need to be copied).

How to specify the file names is the biggest problem.

[nicferrier]

The files stuff is done. elnode-log-access and elnode-error both now go through a general buffer logging function that constantly writes the buffer.