Note that the error log message size must be strictly smaller than
the message buffer size or the message will be dropped, see
https://github.com/epics-base/epics-base/blob/91e1d0ab8022d610bb12b5536a7a6a21644d47f7/modules/libcom/src/error/errlog.c#L130

In EPICS base, the errorlog functionality is initialised with a default
maximum error log size of 256. This means that error messages longer than
that size show as `<<TRUNCATED>>´, which means that we miss a number of
important messages, in particular error messages due to missing symbols
from shared libraries when they are loaded.

`errlogInit` unfortunately is a bit of a singleton; it can only be called
a single time, and it is called when we load require the very first time.
This fix allows us to configure error log buffer sizes at IOC startup time.

When using os.system it calls system() which is hard coded to use '/bin/sh -c':
https://linux.die.net/man/3/system
This causes an issue since '[[ -t 1 ]]' is meant to be executed in bash
and sh fails to execute that command.
This is fixed by telling os.system to execute it via bash.

Note that this is the same as the original bash implementation, and
fixes the terminal even on segfaults.

This only came up if I ran, for example
```
$ echo exit | iocsh
```

See https://unix.stackexchange.com/a/26782/445931. This assume bash,
which is safe, and fixes the issue.

The entry point for the e3 IOC shell has been rewritten from being a shell/bash
script to instead be in python. Together with this comes a plethora of changes -
see the CHANGELOG for a more comprehensive description.