ConnectionExists: reusing possible HTTP+NTLM connections better

Make sure that the special NTLM magic we do is for HTTP+NTLM only since
that's where the authenticated connection is a weird non-standard
paradigm.

Regression brought in 8ae35102c (curl 7.35.0)

Bug: http://curl.haxx.se/mail/lib-2014-02/0100.html
Reported-by: Dan Fandrich
RELEASE-NOTES: synced with 378af08c992
Curl_urldecode: don't allow NULL as receiver

For a function that returns a decoded version of a string, it seems
really strange to allow a NULL pointer to get passed in which then
prevents the decoded data from being returned!

This functionality was not documented anywhere either.

If anyone would use it that way, that memory would've been leaked.

Bug: https://github.com/bagder/curl/pull/90
Reported-by: Arvid Norberg
dict: fix memory leak in OOM exit path

Bug: https://github.com/bagder/curl/pull/90
valgrind: added suppression on optimized code

gcc 4.7.2 with -O2 will optimize Curl_connect by inlining some
functions two levels deep, which makes the valgrind suppression
fail to match. The underlying reason for these idna suppressions is
a gcc strlen optimization when compiling libidn; compiling it with
-fno-builtin-strlen makes this suppression unnecessary.
OS400: Add new options to RPG binding.
curl_easy_setopt.3: Fix word order of CURLOPT_PROXY section

The word CURLOPT_PROXYPORT became detached from its sentence when the
note about the default was added.