manage shell history properly

Some shells (sh/csh/bash) manage history to some extent, but the support isn't very good. Specifically, bash history is stored in ram and isn't written to the histfile until a shell is closed. A new rez-env shell therefore doesn't contain recent command history.

A good fix for bash shell would be to include this step in rez shell startup:

```
# note the leading space. This causes the "history -a" cmd itself to not be stored to history
history -a
```

Wrt other shells (see https://github.com/nerdvegas/rez/issues/1234 ), there is no support for managing history at all.

Start by locating Rez's shell startup logic and read issue #1234 for the related history behavior in other shells. Check how bash, sh, and csh startup paths are handled; done means recent bash history is available in a new rez-env shell and history support for the other named shells is addressed or clearly scoped.

Written by the indexing model from the issue text.