Requested by Igor Sysoev.

It was working for nginx's own 206 replies as they are seen as 200 in the
headers filter module (range filter goes later in the headers filter chain),
but not for proxied replies.

Additional parsing logic added to correctly handle RFC 3986 compliant IPv6 and
IPvFuture characters enclosed in square brackets.

The host validation was completely rewritten. The behavior for non IP literals
was changed in a more proper and safer way:

 - Host part is now delimited either by the first colon or by the end of string
   if there's no colon. Previously the last colon was used as delimiter which
   allowed substitution of a port number in the $host variable.
   (e.g. Host: 127.0.0.1:9000:80)

 - Fixed stripping of the ending dot in the Host header when the host was also
   followed by a port number.
   (e.g. Host: nginx.com.:80)

 - Fixed upper case characters detection. Previously it was broken which led to
   wasting memory and CPU.

Patch by Zaur Abasmirzoev.

If process exited abnormally while holding lock on some shared memory zone -
unlock it.  It may be not safe thing to do (as crash with lock held may
result in corrupted shared memory structure, and other processes will
subsequently crash while trying to access shared data), therefore complain
loudly if unlock succeeds.

It is currently used from master process on abnormal worker termination to
unlock accept mutex (unlocking of accept mutex was broken in 1.0.2).  It is
expected to be used in the future to unlock other mutexes as well.

Shared mutex code was rewritten to make this possible in a safe way, i.e.
with a check if lock was actually held by the exited process.  We again use
pid to lock mutex, and use separate atomic variable for a count of processes
waiting in sem_wait().