MINOR: debug: read all libs in memory when set-dumpable=libs

MINOR: debug: read all libs in memory when set-dumpable=libs

When "set-dumpable" is set to "libs", in addition to marking the process
dumpable, haproxy also reads the binary and shared objects into memory as
a tar archive in a page-aligned location so that these files are easily
extractable from a future core dump. The goal here is to always have
access to the exact same binary and libs as those which caused the core
to happen. It's indeed very frequent to miss some of these, or to get
mismatching files due to a local update that didn't experience a reload,
or to get those of a host system instead of the container.

The in-memory tar file presents everything under a directory called
"core-%d" where %d corresponds to the PID of the worker process. In
order to ease the finding of these data in the core dump, the memory
area is contiguous and surrounded by PROT_NONE pages so that it appears
in its own segment in the core file. The total size used by this is a
few tens of MB, which is not a problem on large systems.