The ngx_wsasend_chain() and ngx_wsarecv_chain() functions were
modified to use only preallocated memory, and the number of
preallocated wsabufs was increased to 64.

In d1bde5c3c5d2, the number of preallocated iovec's for ngx_readv_chain()
was increased.  Still, in some setups, the function might allocate memory
for iovec's from a connection pool, which is only freed when closing the
connection.

The ngx_readv_chain() function was modified to use only preallocated
memory, similarly to the ngx_writev_chain() change in 8e903522c17a.

Control characters (0x00-0x1f, 0x7f) and space are not expected to appear
in the Host header.  Requests with such characters in the Host header are
now unconditionally rejected.

In 71edd9192f24 logging of invalid headers which were rejected with the
NGX_HTTP_PARSE_INVALID_HEADER error was restricted to just the "client
sent invalid header line" message, without any attempts to log the header
itself.

This patch returns logging of the header up to the invalid character and
the character itself.  The r->header_end pointer is now properly set
in all cases to make logging possible.

The same logging is also introduced when parsing headers from upstream
servers.

Control characters (0x00-0x1f, 0x7f), space, and colon were never allowed in
header names.  The only somewhat valid use is header continuation which nginx
never supported and which is explicitly obsolete by RFC 7230.

Previously, such headers were considered invalid and were ignored by default
(as per ignore_invalid_headers directive).  With this change, such headers
are unconditionally rejected.

It is expected to make nginx more resilient to various attacks, in particular,
with ignore_invalid_headers switched off (which is inherently unsecure, though
nevertheless sometimes used in the wild).

Control characters (0x00-0x1f, 0x7f) were never allowed in URIs, and must
be percent-encoded by clients.  Further, these are not believed to appear
in practice.  On the other hand, passing such characters might make various
attacks possible or easier, despite the fact that currently allowed control
characters are not significant for HTTP request parsing.

From now on, requests with spaces in URIs are immediately rejected rather
than allowed.  Spaces were allowed in 31e9677b15a1 (0.8.41) to handle bad
clients.  It is believed that now this behaviour causes more harm than
good.

Per RFC 3986 only the following characters are allowed in URIs unescaped:

unreserved    = ALPHA / DIGIT / "-" / "." / "_" / "~"
gen-delims    = ":" / "/" / "?" / "#" / "[" / "]" / "@"
sub-delims    = "!" / "$" / "&" / "'" / "(" / ")"
              / "*" / "+" / "," / ";" / "="

And "%" can appear as a part of escaping itself.  The following
characters are not allowed and need to be escaped: %00-%1F, %7F-%FF,
" ", """, "<", ">", "\", "^", "`", "{", "|", "}".

Not escaping ">" is known to cause problems at least with MS Exchange (see
http://nginx.org/pipermail/nginx-ru/2010-January/031261.html) and in
Tomcat (ticket #2191).

The patch adds escaping of the following chars in all URI parts: """, "<",
">", "\", "^", "`", "{", "|", "}".  Note that comments are mostly preserved
to outline important characters being escaped.

After 4954530db2af, the ";" character is escaped by
ngx_escape_uri(NGX_ESCAPE_ARGS).