Support installation via $E9_HOME

[crhf]

run e9syscall everywhere, instead of in this directory only

[GJDuck]

This is a good idea. However, rather than expecting the user to define an $E9_HOME environment variable, a better approach would be for the e9syscall script to automatically find its own install path, then search for the e9patch binary using this path. E9AFL uses this approach.