Add a modifier to output various os-level tunables

[linsword13]

Example output:

default (null) context figures of merit:
    possible hostname = linsword-13.c.googlers.com
    modifier::tunables::address-space-randomization = 2
    modifier::tunables::numa-balancing = 0
    modifier::tunables::smt-active = 1
    modifier::tunables::thp-enabled = always
    modifier::tunables::thp-defrag = madvise
    modifier::tunables::explicit-hugepage-count = 0
    modifier::tunables::explicit-hugepage-size = 2048 kB