Cleanup & fix shell-quoting in generated 'env' and 'run-agent.sh' scripts

[dlitz]

• Fix shell-quoting of arguments in .gnupg/trezor/env and .gnupg/trezor/run-agent.sh for non-Windows platforms.

• Fix AttributeError: 'float' object has no attribute 'translate' when passing --cache-expiry-seconds to *-gpg init on all platforms.

• Add "exec" to the sh scripts, where appropriate, so that the wrapped command replaces the shell, rather than running in a subprocess.

• Move Bourne sh and Windows batch-file templates into separate sections of code, since it doesn't make sense to combine them once we're doing quoting properly.

• Use standard POSIX /bin/sh syntax in the env script so that this has the possibility of working on systems without /bin/bash.