Note: Linux only

Core dumps of large buffer pool pages take time and space
and pose potential data expose in scenarios where data-at-rest
encryption is deployed.

Here we use madvise(MADV_DONT_DUMP) on large memory allocations
used by the innodb buffer pool, log_sys and recv_sys. The effect
of this system call is that these memory areas will not appear in
a core dump. Data from these buffers is rarely useful in fault
diagnosis.

log_sys and recv_sys structures now use large memory allocations
for their large buffer.

Debug builds don't include the madvise syscall and as such will
include full core dumps.

A function, buf_madvise_do_dump, is added but never called. It
is there to be called from a debugger to re-enable the core
dumping of all of these pages if for some reason the entire
contents of these buffers are needed.

Idea thanks to Hartmut Holzgraefe