BUG/MINOR: init: fix default global settings being overwritten by -G

BUG/MINOR: init: fix default global settings being overwritten by -G

The default global configuration tuning settings (tune.memory.hot-size,
expose-experimental-directives, and tune.pipesize) were lost when the
-G option was used.

The bug was caused by an incorrect scope: these default settings were
nested inside the block that generates the default global header, which
is skipped/overwritten when -G is provided. Fix this by closing the
conditional block early.

This patch depends on this commit:
    "01f4e33ea MINOR: hbuf: new lightweight hbuf API"

Must be backported to 3.4.