[MEDIUM] splice: add configuration options and set global.maxpipes

[MEDIUM] splice: add configuration options and set global.maxpipes

Three new options have been added when CONFIG_HAP_LINUX_SPLICE is
set :
  - splice-request
  - splice-response
  - splice-auto

They are used to enable splicing per frontend/backend. They are also
supported in defaults sections. The "splice-auto" option is meant to
automatically turn splice on for buffers marked as fast streamers.
This should save quite a bunch of file descriptors.

It was required to add a new "options2" field to the proxy structure
because the original "options" is full.

When global.maxpipes is not set, it is automatically adjusted to
the max of the sums of all frontend's and backend's maxconns for
those which have at least one splice option enabled.