[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/bootsup.c

@@ -19,7 +19,7 @@
 #include "bootcode.h"
 #include "fsutil.h"
 
-#include "setuplib.h" // HAXX for IsUnattendedSetup!!
+#include "setuplib.h" // HACK for IsUnattendedSetup
 
 #include "bootsup.h"
 
@@ -1658,6 +1658,7 @@ GetDeviceInfo(
  * @return  An NTSTATUS code indicating success or failure.
  **/
 NTSTATUS
+NTAPI
 InstallBootManagerAndBootEntries(
     _In_ ARCHITECTURE_TYPE ArchType,
     _In_ PCUNICODE_STRING SystemRootPath,
@@ -1813,6 +1814,7 @@ Quit:
 }
 
 NTSTATUS
+NTAPI
 InstallBootcodeToRemovable(
     _In_ ARCHITECTURE_TYPE ArchType,
     _In_ PCUNICODE_STRING RemovableRootPath,