Notably, this fixes CAPA and AUTH output.

The bug had appeared in nginx 1.11.6 (73b451d304c0).

Most notably, warning W8012 (comparing signed and unsigned values) reported
in multiple places where an unsigned value of small type (e.g., u_short) is
promoted to an int and compared to an unsigned value.

Warning W8072 (suspicious pointer arithmetic) disabled, it is reported
when we increment base pointer in ngx_shm_alloc().

This is needed to allow TLS client certificate auth to work. With
ssl_verify_client configured, the auth daemon can choose to allow the
connection to proceed based on the certificate data.

This has been tested with Thunderbird for IMAP only. I've not yet found a
client that will do client certificate auth for POP3 or SMTP, and the method is
not really documented anywhere that I can find. That said, its simple enough
that the way I've done is probably right.

External structures are now identical regardless of mail SSL module
compiled in or not.

The IPV6_V6ONLY macro is now checked only while parsing appropriate flag
and when using the macro.

The ipv6only field in listen structures is always initialized to 1,
even if not supported on a given platform.  This is expected to prevent
a module compiled without IPV6_V6ONLY from accidentally creating dual
sockets if loaded into main binary with proper IPV6_V6ONLY support.

All the errors that prevent loading configuration must be printed on the "emerg"
log level.  Previously, nginx might silently fail to load configuration in some
cases as the default log level is "error".

The new type ngx_uint_t was supposed when formatting the line number.