[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/utils/osdetect.c

@@ -209,12 +209,16 @@ EnumerateInstallations(
     return STATUS_SUCCESS;
 }
 
-/*
- * FindSubStrI(PCWSTR str, PCWSTR strSearch) :
- *    Searches for a sub-string 'strSearch' inside 'str', similarly to what
- *    wcsstr(str, strSearch) does, but ignores the case during the comparisons.
- */
-PCWSTR FindSubStrI(PCWSTR str, PCWSTR strSearch)
+/**
+ * @brief
+ * Finds the first occurrence of a sub-string 'strSearch' inside 'str',
+ * using case-insensitive comparisons.
+ **/
+PCWSTR
+NTAPI
+FindSubStrI(
+    _In_ PCWSTR str,
+    _In_ PCWSTR strSearch)
 {
     PCWSTR cp = str;
     PCWSTR s1, s2;
@@ -760,6 +764,7 @@ FindNTOSInstallations(
  **/
 // EnumerateNTOSInstallations
 PGENERIC_LIST
+NTAPI
 CreateNTOSInstallationsList(
     _In_ PPARTLIST PartList)
 {