Move debootstrap to default package list on judgehosts.
Restrict memory during compilation phase with new limit.

Notice that the memory limit during compilation phase should be at
least as high as during executation phase. Furthermore, ld needs a
large amount of memory for huge static arrays.
Introduce new filesize limit for compilation phase.

The default filesize limit for submissions (4MB) is too low even for
small test programs. g++ (with -pipe) creates a larger file during
compilation even if the final result is 1.5MB.