[RFC] mkmux: avoid spiky writes and waste of page cache space

while working on a more recent version for the raspberry pi I found that the following change is very useful if the system (with limited memory and CPU resources) is used as DVR. maybe its a tweak that is useful in general. i do not know if the below part is also used when doing timeshift in this case the POSIX_FADV_DONTNEED might be bad. but maybe its possible to detect this.