Support credential setting for shells other than bash

[nonspecialist]

Right now, credulous emits statements to set environment variables in bash/zsh -- the shells are widespread, but not the only ones in use.

We should also support a range of other popular shells, in a way that makes it easy to extend the syntaxes we emit.

At first glance, we should probably also handle:

• vanilla Bourne shell (no export FOO=bar syntax)
• ash and its variants (same syntax as Bourne)
• Windows Powershell (yes, Windows, though I don't know the mechanism)
• csh/tcsh etc (though there are religious debates about the use of those)

Actually, supporting bash/zsh/vanilla Bourne/ash etc could be done by just changing the emitted text to be:

FOO=bar; export FOO

instead of:

export FOO=bar

because that syntax is supported by all the bourne-originated shells.