This commit only affects the library behaviour unless
nghttp2_option_set_no_http_messaging() is used.

We like strict validation against header field name and value against
RFC 7230, but we have already so much web sites and libraries in
public internet which do not obey these rules.  Simply just
terminating stream because of this may break web sites and it is too
disruptive.  So we decided that we should be conservative here so
those header fields containing illegal characters are just ignored.
But we are conservative only for regular headers.  We are strict for
pseudo headers since it is new to HTTP/2 and new implementations
should know the rules better.

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.