Add support to extra environment variables in module `avocado.utils.process`.

The main `SubProcess` class now supports an argument (`env`) to receive extra environment variables to the subprocess execution.

Note that it's extra in the sense that it includes the current environment variables, so the previous environment is not destroyed.

The utility functions now accepts this extra environment variables (through the `env` argument). It's more robust to pass variables as a dictionary than using the shell feature of shell variables. The module `avocado.utils.build` have been updated to use this feature. Here's an example:

```
avocado.utils.build.make(self.srcdir, env={'CFLAGS': '-g -O0'}, extra_args='synctest')
```