Revert changes relative to lib/*.[ch] recent renaming

This reverts renaming and usage of lib/*.h header files done
28-12-2012, reverting 2 commits:

  f871de0... build: make use of 76 lib/*.h renamed files
  ffd8e12... build: rename 76 lib/*.h files

This also reverts removal of redundant include guard (redundant thanks
to changes in above commits) done 2-12-2013, reverting 1 commit:

  c087374... curl_setup.h: remove redundant include guard

This also reverts renaming and usage of lib/*.c source files done
3-12-2013, reverting 3 commits:

  13606bb... build: make use of 93 lib/*.c renamed files
  5b6e792... build: rename 93 lib/*.c files
  7d83dff... build: commit 13606bbfde follow-up 1

Start of related discussion thread:

  http://curl.haxx.se/mail/lib-2013-01/0012.html

Asking for confirmation on pushing this revertion commit:

  http://curl.haxx.se/mail/lib-2013-01/0048.html

Confirmation summary:

  http://curl.haxx.se/mail/lib-2013-01/0079.html

NOTICE: The list of 2 files that have been modified by other
intermixed commits, while renamed, and also by at least one
of the 6 commits this one reverts follows below. These 2 files
will exhibit a hole in history unless git's '--follow' option
is used when viewing logs.

  lib/curl_imap.h
  lib/curl_smtp.h

src/tool_setup.h

@@ -25,15 +25,15 @@
 #define CURL_NO_OLDIES
 
 /*
- * curl_setup.h may define preprocessor macros such as _FILE_OFFSET_BITS and
+ * setup.h may define preprocessor macros such as _FILE_OFFSET_BITS and
  * _LARGE_FILES in order to support files larger than 2 GB. On platforms
  * where this happens it is mandatory that these macros are defined before
  * any system header file is included, otherwise file handling function
  * prototypes will be misdeclared and curl tool may not build properly;
- * therefore we must include curl_setup.h before curl.h when building curl.
+ * therefore we must include setup.h before curl.h when building curl.
  */
 
-#include "curl_setup.h" /* from the lib directory */
+#include "setup.h" /* from the lib directory */
 
 /*
  * curl tool certainly uses libcurl's external interface.
@@ -67,7 +67,7 @@
 #endif
 
 #ifndef HAVE_STRDUP
-#  include "curl_strdup.h"
+#  include "strdup.h"
 #  define strdup(ptr) curlx_strdup(ptr)
 #endif