Add option to write URL rejections to a tab-delimited CSV log.

* main.c: Add "--rejected-log" option.
 * init.c: Add "rejectedlog" command.
 * options.h: Add "rejected_log" parameter string.
 * wget.texi: Add brief documentation on new --rejected-log option.
 * recur.c: Optionally log details of URLs not traversed.
   Add reject_reason enum.
   (download_child_p -> download_child): Return a reject_reason.
   (descend_redirect_p -> descend_redirect): Return a reject_reason.
   (retrieve_tree): Support logging reasons for rejection.
   Add write_reject_log_header that writes a CSV format header to a file.
   Add write_reject_log_url that writes a url struct to a file in CSV format.
   Add write_reject_log_reason that writes the URL and parent URL as well as the
   rejection reason to a CSV file.
 * Test--rejected-log.px: Add a basic test for the --rejected-log command.
 * tests/Makefile.am: Run Test--rejected-log.px.

This allows you to figure out why URLs are being rejected and some context
around it. CSV is used as the output format since it can be used easily parsed,
it's delimited by tabs instead of commas to allow using all (quoted) URL
characters and includes column names which may be used for compatibility.

doc/wget.texi

@@ -551,6 +551,11 @@ would be resolved to @samp{http://foo/baz/b.html}.
 @cindex specify config 
 @item --config=@var{FILE}
 Specify the location of a startup file you wish to use.
+
+@item --rejected-log=@var{logfile}
+Logs all URL rejections to @var{logfile} as comma separated values.  The values
+include the reason of rejection, the URL and the parent URL it was found in.
+
 @end table
 
 @node Download Options, Directory Options, Logging and Input File Options, Invoking