Suggested-by: Rainer Canavan

Closes #2126

This reverts commit 9ffad8eb.

It was actually added rather recently in 8e8afa82 due to a crash
that would otherwise happen in the RTSP code. As I don't think we've
fixed that behavior yet, we better keep this work-around until we have
fixed it better.

Prune the DNS cache immediately after the dns entry is unlocked in
multi_done. Timed out entries will then get discarded in a more orderly
fashion.

Test506 is updated

Reported-by: Oleg Pudeyev

Fixes #2169
Closes #2170

Prior to this change SSLKEYLOGFILE used line buffering on WIN32 just
like it does for other platforms. However, the Windows CRT does not
actually support line buffering (_IOLBF) and will use full buffering
(_IOFBF) instead. We can't use full buffering because multiple processes
may be writing to the file and that could lead to corruption, and since
full buffering is the only buffering available this commit disables
buffering for Windows SSLKEYLOGFILE entirely (_IONBF).

Ref: https://github.com/curl/curl/pull/1346#issuecomment-350530901

... to help users understand what is or isn't done!

Closes #1900

- Change "never does nothing" double-negative to "never does anything".

Closes https://github.com/curl/curl/pull/2168

These are OS/2-specific things added to the code in the year 2000. They
were always ugly. If there's any user left, they still don't need it
done this way.

Closes #2166

- Allow proxy_ssl to be checked for pending data even when connssl does
  not yet have an SSL handle.

This change is for posterity. Currently there doesn't seem to be a code
path that will cause a pending data check when proxyssl could have
pending data and the connssl handle doesn't yet exist [1].

[1]: Recall that an https proxy connection starts out in connssl but if
the destination is also https then the proxy SSL backend data is moved
from connssl to proxyssl, which means connssl handle is temporarily
empty until an SSL handle for the destination can be created.

Ref: https://github.com/curl/curl/commit/f4a6238#commitcomment-24396542

Closes https://github.com/curl/curl/pull/1916

That data is only ever used by the CURLOPT_INTERLEAVEFUNCTION callback
and that option isn't set or used by the curl tool!

Updates the 9 tests that verify --libcurl

Closes #2167

... error messages are _not_ sent to stderr if this is not set.

Fixes #2076
Closes #2125

Make it use a max 10Hz update frequency for this case as well. Return
early if the "point" hasn't moved since last invoke.

Reported-by: Elliot Saba

Fixes #2158
Closes #2163

...the former link is dead.

Reported-by: Frank Gevaerts

Fixes #2088
Closes #2157

... to allow build on older Linux dists (specifically CentOS 4.8 on gcc
4.8.5)

Closes #2160

... to aid debugging openldap library using its built-in debug messages.

Closes #2159

... due it relying on pthreads to link.

... and bump next release version to 7.58.0

... as in "file://c:\some\path\curl.out"

Reviewed-by: Matthew Kerwin
Closes #2154

Needed by HPE NonStop NSE and NSX systems

Fixes #2146
Closes #2155

Lists all SSL backends that were enabled at build-time.

Suggested-by: Oleg Pudeyev
Fixes #2128

Connections that are used for HTTP/1.1 Pipelining or HTTP/2 multiplexing
only get additional transfers added to them if the existing connection
is held by the same multi or easy handle. libcurl does not support doing
HTTP/2 streams in different threads using a shared connection.

Closes #2152

If the lock is released before the dealings with the bundle is over, it may
have changed by another thread in the mean time.

Fixes #2132
Fixes #2151
Closes #2139

... by removing a superfluous NULL pointer check that also confuses
Coverity.

Fixes #2143
Closes #2153

For pop3/imap/smtp, added test 891 to somewhat verify the pop3
case.

For this, I enhanced the pingpong test server to be able to send back
responses with LF-only instead of always using CRLF.

Closes #2150

Figured out while reviewing code in the libssh backend. The pointer was
checked for NULL after having been dereferenced, so we know it would
always equal true or it would've crashed.

Pointed-out-by: Nikos Mavrogiannopoulos

Bug #2143
Closes #2148