This change is motivated by that fact that we use it for UNIX domain
socket as well as IPv4.

This commit also fixes environment variables used to tell inherited
file descriptors to new binary are stacked up each time new binary is
executed.

This is typically programming mistake that we did not check return
value before doing another.

Use struct stat.st_mtime instead of st_mtim which seems to be Linux
specific.

This API function with nonzero |val| parameter disables HTTP Messaging
validation in nghttp2 library, so that application can use nghttp2
library for non-HTTP use.