Option to only profile MPI

[MeisterEule]

Some users are only interested in the MPI performance of their code, not the detailed function performance.
In these cases, it is sufficient to profile MPI, as it can be achieved by linking Vftrace without function instrumentation (although then no output is generated). There should be an option to return from vftr_function_entry and _exit if no MPI function is present.

[MeisterEule]

This already works if you do not compile with -finstrument-functions, since MPI functions are always instrumented. I would not implement an additional option to obtain this functionality with instrumentation of all functions, in order to reduce code overhead.