Set -M, -H, -c and other memory-related values based on available virtual/physical memory

From John Meacham:

perhaps if -M is not otherwise set, getrlimit(RLIMIT_AS,..) could be called and the maximum heap size set to just under that, since it is the point that the OS will forcefully kill the program anyway.

Ravi Nanavati would like to be able to set the value to a percentage of physical RAM, e.g. `prog +RTS -H256m -M95% -RTS'.

Bulat Ziganshin would like to be able to do the same with -c\.