Implement different policies / handlers based on environment variables

[rbertran]

Enable a configuration API through environment variables.

We would like to do easily the following types of experiments (copied from a note I sent some time ago):

• the application executed by the profiler (baseline + tracing overhead)
• the application executed by the profiler + only inserting traps (baseline + tracing overhead + trapping overhead)
• the application executed by the profiler + inserting traps + protect all + unprotect all on 1st failure (baseline + tracing overhead + trapping overhead + minimum tracing overhead)
• the application executed by the profiler + inserting traps + protect all + unprotect the page on failures (baseline + tracing overhead + trapping overhead + tracing overhead)
• the application executed by the profiler + inserting traps + protect all + do not unprotect the page on failures (baseline + tracing overhead + trapping overhead + maximum tracing overhead)
• the application executed by the profiler + inserting traps + protect all + unprotect the page on failures and write to disk (all IO)

To do so, we should be able to control which of these 'modes' to use using either a command line argument or a environment variable. The main profiler/driver will select the handler based on that.