Configure/build changes and fixes:

*) Revamped "configure --help" text.

*) FreeBSD 10-current has recently gotten POSIX_FADV_* macros.
   A fix for the broken build applied.

*) AIX 7 defines sys_nerr in errno.h, therefore <errno.h> included
   in the sys_nerr test.

   When sys_nerr and _sys_nerr are missed on a particular platform
   and our euristic for a maximum errno detection applied always
   print the maximum errno number we reached instead of printing void.

   This makes possible to build nginx on AIX 7.1.

*) Made it possible to build/install from the SVN checkout.

Manpage changes:

*) Support link.

*) Commented out reference to non-existing nginx.conf(5).

*) Rebuild manpage only if needed.

error_log related fixes:

*) Complain on invalid log levels.

   Previously only first log level was required to be correct, while
   error_log directive in fact accepts list of levels (e.g. one may
   specify "error_log ... debug_core debug_http;").  This resulted
   in (avoidable) wierd behaviour on missing semicolon after error_log
   directive, e.g.

       error_log /path/to/log info
       index index.php;

   silently skipped index directive and it's arguments (trying to
   interpret them as log levels without checking to be correct).

*) Fixed configuration summary and manpage contents for the special
   --error-log-path=stderr case.