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

tests/libtest/CMakeLists.txt

@@ -5,7 +5,7 @@ function(SETUP_TEST TEST_NAME)          # ARGN are the files in the test
   string(TOUPPER ${TEST_NAME} UPPER_TEST_NAME)
 
   include_directories(
-    ${CURL_SOURCE_DIR}/lib          # To be able to reach "curl_setup_once.h"
+    ${CURL_SOURCE_DIR}/lib          # To be able to reach "setup_once.h"
     ${CURL_BINARY_DIR}/lib          # To be able to reach "curl_config.h"
     ${CURL_BINARY_DIR}/include      # To be able to reach "curl/curlbuild.h"
     )