Add process struct

[vinc]

Add a process struct to store the current dir, user, and env vars.

This will allow the use of relative paths and of a home dir set in env vars.

The go command, equivalent to the cd in Unix and Windows, can now be used. With go <dir> to change the current dir and go without argument to print the current dir.

[vinc]

At the moment there is only one process but this will change.

[vinc]

Use goto instead of go for the fullname command, and still use g for its short name. It could of course be aliased to cd.

> goto /tmp

> list
alice.txt
hello.txt

> read hello.txt
Hello, world!