Use portable inet_pton() for all Win32 builds.

Rename the portable inet_pton() to xp_inet_pton() and define a wrapper macro (for all Widows builds) to use it instead of inet_pton() as not all supported Windows systems (e.g. WinXP) have an inet_pton() implementation.

This allows the HAProxy mod to be re-enabled by default without preventing executing on WinXP.