curl mustn't try to use the control connection after the 421 is received

I found a bug in tftp_tx() in tftp.c. If a data resend is done after
reception of an ACK/OACK, the call to sendto is wrong.

Instead of reopening the downloaded file, fsetxattr uses the (already
open) file descriptor to attach extended attributes. This makes the
procedure more robust against errors caused by moved or deleted files.

Bug: https://bugzilla.redhat.com/650255
Reported by: Simon H.

I also documented the filtering logic in the symbol-scan.pl function to
clarify why not all CURL_* symbols are included.

... and do not send ABOR unless really necessary.

Bug: https://bugzilla.redhat.com/649347
Reported by: Simon H.

I also recounted and updated the command line and libcurl options.

My copy and paste job was a little too much copy and I missed to adjust
it properly to sys/xattr.h all over and this is a fix to cure that.

CURLOPT_RESOLVE is a new option that sends along a curl_slist with
name:port:address sets that will populate the DNS cache with entries so
that request can be "fooled" to use another host than what otherwise
would've been used. Previously we've encouraged the use of Host: for
that when dealing with HTTP, but this new feature has the added bonus
that it allows the name from the URL to be used for TLS SNI and server
certificate name checks as well.

This is a first change. Surely more will follow to make it decent.

Removed the code that was needed for libcurl before 7.19.0 which now is
more than two years old.

Simplified the top comment and corrected the URL.

If the query result has a binary attribute, the binary attribute is
base64 encoded. But all following non binary attributes are also base64
encoded which is wrong.

This is a test (LDAP server is public).

curl
ldap://x500.bund.de:389/o=Bund,c=DE?userCertificate,certificateSerialNumber?sub
?cn=*Woehleke*

setxattr is a glibc call to set extended attributes, so configure now
checks for it and the code is adapted to only build when the
functionality is present.

It is often convinient to track back the source of a once downloaded
file; this patch makes curl store the source URL and other metadata
alongside the retrieved file by using the extended attributes (if
supported by the file system and enabled by --xattr).

Test 580 is removed again for two reasons:

1) Some compilers aren't satisfied by just a data variable called 'test'
when first.o wants a function called 'test'. The Solaris compiler says
"ld: warning: symbol `test' has differing types:" while the AIX compiler
downright rejects it.

2) Test case 1119 that was added after this test is way more complete
and cover everything test 580 does and more without introducing the same
problems.

This reverts commit b0fd03f5,
4b2fbe1e, afecd1aa, 68cde058

262 - Manual setting of TLS Server Name Indication - use Host:

If you use a custom Host: name in a request to a SSL server, libcurl
will now use that given name when it verifies the server certificate to
be correct rather than using the host name used in the actual URL.

The redirect check is already done at the position where the customhost
field is assigned so there's no point in doing that a second time.

When given a custom host name in a Host: header, we can use it for
several different purposes other than just cookies, so we rename it and
use it for SSL SNI etc.

OpenSSL SNI host name should be set to the custom Host header, if the
user provided one.

I've made the code intended using curl-style now to look more like other
examples.

My previous "fix" was a bit too invasive but is now fixed again.