[SETUPLIB][REACTOS][USETUP] Turn setuplib into a DLL shared between TUI and GUI 1st-stage setups (#7523)

CORE-13525

Notes:
- Most of the exported functions have been turned from default cdecl to explicit stdcall / "NTAPI".
- The two InitializeSetup() phases have been collapsed to make the initialization simpler.

Average reductions (percentages; see PR #7523 for actual numbers):

x86 Debug builds:
reactos.exe: 35.1%
smss.exe   : 39.8%
Total (including setuplib.dll): 17.9%

x86 Release builds:
reactos.exe: 22.3%
smss.exe   : 25.0%
Total (including setuplib.dll): 10.6%

x64 Debug builds:
reactos.exe: 40.6%
smss.exe   : 41.6%
Total (including setuplib.dll): 20.0%

x64 Release builds:
reactos.exe: 22.8%
smss.exe   : 22.3%
Total (including setuplib.dll): 10.1%

base/setup/lib/precomp.h

@@ -29,10 +29,14 @@
 
 #include <ntstrsafe.h>
 
-
 /* Filesystem headers */
 #include <reactos/rosioctl.h>   // For extra partition IDs
 
+
+#ifndef SPLIBAPI
+#define SPLIBAPI
+#endif
+
 //
 ///* Internal Headers */
 //#include "interface/consup.h"
@@ -51,7 +55,6 @@
 //#include "filesup.h"
 //#include "genlist.h"
 
-
 extern HANDLE ProcessHeap;
 
 #include "errorcode.h"