MEDIUM: fd: Make it possible to have one fdtab per thread-group

MEDIUM: fd: Make it possible to have one fdtab per thread-group

Add a new global flag, GTUNE_NO_TG_FD_SHARING. When set, the goal is to
have to a different file descriptor table for each thread group, if
the platform supports separate file descriptor tables for each thread
(so right now, only linux with the unshare() syscall), as with many
threads, the kernel lock for the file descriptor table is a huge source
of contention.
This commit just makes it so if the flag is set, we will have one fdtab
per thread group, and we will call unshare() for each thread group, so
that each has its own file descriptor table.
Note that for now there is no way to set the flag, as a lot more work is
needed before the feature is usable. As such, this commit should have no
noticeable impact.