Add defer_accept option to enable TCP_DEFER_ACCEPT

Merge: 6fb5e9636fa1fffc62e86561e7dcd734fa2ee53f

Add defer_accept option to enable TCP_DEFER_ACCEPT
Charybdis commits referenced:

atheme/charybdis@77cb59b
    libratbox: add support for TCP_DEFER_ACCEPT on linux

atheme/charybdis@aa4737a
    libratbox: make defer_accept optional.

atheme/charybdis@02270e9
    Add listen::defer_accept option for controlling usage
    of TCP_DEFER_ACCEPT option.

atheme/charybdis@797a29f
    libratbox: Allow defer_accept on FreeBSD.
Merge branch 'defer-accept' into devel