Convert some mdrun utility code to C++

Eliminated unused variables, used std::min/max, renamed "try" variable
to "attempt" to avoid reserved word, moved declaration of MPI-specific
variables, added space between string literals intended for
concatenation, moved some variable declarations inside matching
preprocessor regions.

Change-Id: Id16fea81d90f15de9a8e8e6eadeb364df954fff7