mem: make segment preallocation OS-specific

mem: make segment preallocation OS-specific

In the perfect world, it wouldn't matter how much memory was
preallocated because most of it was always going to be private
anonymous zero-page mappings for the duration of the program.
However, in practice, due to peculiarities of FreeBSD, we need
to additionally limit memory allocation there. This patch moves
the segment preallocation to EAL private functions that will be
implemented by an OS-specific EAL rather than being in the common
memory-related code.

Since there is no support for growing/shrinking memory use at
runtime on FreeBSD anyway, this does not inhibit any functionality
but makes core dumps faster even on default settings.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>