md5 & metalink: use better build macros on Apple operating systems

Previously we used __MAC_10_X and __IPHONE_X to mark digest-generating
code that was specific to OS X and iOS. Now we use
__MAC_OS_X_VERSION_MAX_ALLOWED and __IPHONE_OS_VERSION_MAX_ALLOWED
instead of those macros.

Bug: http://sourceforge.net/p/curl/bugs/1255/
Reported by: Edward Rudd
darwinssl: fix build error in crypto authentication under Snow Leopard

It turns out Snow Leopard not only has SecItemCopyMatching() defined in
a header not included by the omnibus header, but it won't work for our
purposes, because searching for SecIdentityRef objects wasn't added
to that API until Lion. So we now use the old SecKeychainSearch API
instead if the user is building under, or running under, Snow Leopard.

Bug: http://sourceforge.net/p/curl/bugs/1255/
Reported by: Edward Rudd
darwinssl: added LFs to some strings passed into infof()

(This doesn't need to appear in the release notes.) I noticed a few places
where infof() was called, and there should've been an LF at the end of the
string, but there wasn't.
curl: make --progress-bar update the line less frequently

Also, use memset() instead of a lame loop.

The previous logic that tried to avoid too many updates were very
ineffective for really fast transfers, as then it could easily end up
doing hundreds of updates per second that would make a significant
impact in transfer performance!

Bug: http://curl.haxx.se/mail/archive-2013-07/0031.html
Reported-by: Marc Doughty
curl: --progress-bar max update frequency now at 5Hz
curl: follow-up for commit 5af2bfb9

Use tvnow() and tvdiff() to avoid introducing new linkage issues
curl: second follow-up for commit 5af2bfb9

Display progress-bar unconditionally on first call
Add in the files needed to build libcurl shared images on VMS.

Update the packages/vms/readme file to be current.

Also some files for the GNV based build were either missing or needed an
update.

curl_crtl_init.c is a special file that is run before main() to
set up the proper C runtime behavior.

generate_vax_transfer.com generates the VAX transfer vector modules from
the gnv_libcurl_symbols.opt file.

gnv_conftest.c_first is a helper file needed for configure scripts to
come up with the expected answers on VMS.

gnv_libcurl_symbols.opt is the public symbols for the libcurl shared
image.

gnv_link_curl.com builds the shared libcurl image and rebuilds other
programs to use it.

macro32_exactcase.patch is a hack to make a local copy of the VMS Macro32
assembler case sensitive, which is needed to build the VAX transfer modules.

report_openssl_version.c is a tool for help verify that the libcurl
shared image is being built for a minium version of openssl.
packages/vms/Makefile.am: add latest file additions to EXTRA_DIST
DOCS: Added IMAP URL example for listing new messages

In addition to listing the folder contents, in the URL examples, added
an example to list the new messages waiting in the user's inbox.
Revert "DOCS: Added IMAP URL example for listing new messages"

This reverts commit 82ab5f1b0c7c3f as this was the wrong place to
document the complexity of IMAP URLs and Custom Requests.