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).

On Linux, SO_REUSEADDR allows completely duplicate UDP sockets, so using
SO_REUSEADDR when testing configuration results in packets being dropped
if there is an existing traffic on the sockets being tested (ticket #2187).
While dropped packets are expected with UDP, it is better to avoid this
when possible.

With this change, SO_REUSEADDR is no longer set on datagram sockets when
testing configuration.

Since nginx always uses exactly one entry in the question section of
a DNS query, and never uses compression pointers in this entry, parsing
of a DNS response in ngx_resolver_process_response() does not expect
compression pointers to appear in the question section of the DNS
response.  Indeed, compression pointers in the first name of a DNS response
hardly make sense, do not seem to be allowed by RFC 1035 (which says
"a pointer to a prior occurance of the same name", note "prior"), and
were never observed in practice.

Added an explicit check to ngx_resolver_process_response()'s parsing
of the question section to properly report an error if compression pointers
nevertheless appear in the question section.

Instead of checking on each label if we need to place a dot or not,
now it always adds a dot after a label, and reduces the resulting
length afterwards.

To make the code easier to read, reworked the ngx_resolver_copy()
copy loop to match the one used to calculate length.  No functional
changes.