Add defer_accept option to enable TCP_DEFER_ACCEPT

Charybdis commits referenced:

  atheme/charybdis@77cb59b319b670289d5c9ca3c26fb6d0870297b3
    libratbox: add support for TCP_DEFER_ACCEPT on linux

  atheme/charybdis@aa4737a0491f7b3f04560a222f0d0fe58c78cc3e
    libratbox: make defer_accept optional.

  atheme/charybdis@02270e96020968fcefc6c2b2f847fe8912f129ae
    Add listen::defer_accept option for controlling usage of
    TCP_DEFER_ACCEPT option.

  atheme/charybdis@797a29f353cbde0ba94d81afcc58ee96025bd753
    libratbox: Allow defer_accept on FreeBSD.