win, build: add arbitrary MSBuild flag option and binlog option

This change adds a 'msbuild_arg' option to vcbuild.bat that can be used
to pass arbitrary flags to MSBuild.

It also adds a 'binlog' flag as a shortcut 'msbuild_arg' option to
enable binary logging to `%config%\node.binlog`. This is especially
convenient when debugging changes to the build system.

In the process of developing this change, the idea of adding 'setlocal'
to the beginning of the script was rejected since other tasks in this
repo rely on the exported environment variables. This change adds a
note describing this.

vcbuild.bat

@@ -1,5 +1,9 @@
 @if not defined DEBUG_HELPER @ECHO OFF
 
+:: Other scripts rely on the environment variables set in this script, so we
+:: explicitly allow them to persist in the calling shell.
+endlocal
+
 if /i "%1"=="help" goto help
 if /i "%1"=="--help" goto help
 if /i "%1"=="-help" goto help
@@ -59,6 +63,7 @@ set no_cctest=
 set cctest=
 set openssl_no_asm=
 set doc=
+set extra_msbuild_args=^ 
 
 :next-arg
 if "%1"=="" goto args-done
@@ -132,6 +137,8 @@ if /i "%1"=="no-cctest"     set no_cctest=1&goto arg-ok
 if /i "%1"=="cctest"        set cctest=1&goto arg-ok
 if /i "%1"=="openssl-no-asm"   set openssl_no_asm=1&goto arg-ok
 if /i "%1"=="doc"           set doc=1&goto arg-ok
+if /i "%1"=="binlog"        set extra_msbuild_args=%extra_msbuild_args% /binaryLogger:%config%\node.binlog&goto arg-ok
+if /i "%1"=="msbuild_arg"   set extra_msbuild_args=%extra_msbuild_args% %2&goto arg-ok-2
 
 echo Error: invalid command line option `%1`.
 exit /b 1
@@ -313,7 +320,7 @@ if "%target%"=="Build" (
   if defined cctest set target="Build"
 )
 if "%target%"=="rename_node_bin_win" if exist "%config%\cctest.exe" del "%config%\cctest.exe"
-msbuild node.sln %msbcpu% /t:%target% /p:Configuration=%config% /p:Platform=%msbplatform% /clp:NoSummary;NoItemAndPropertyList;Verbosity=minimal /nologo
+msbuild node.sln %msbcpu% /t:%target% /p:Configuration=%config% /p:Platform=%msbplatform% /clp:NoSummary;NoItemAndPropertyList;Verbosity=minimal /nologo %extra_msbuild_args%
 if errorlevel 1 (
   if not defined project_generated echo Building Node with reused solution failed. To regenerate project files use "vcbuild projgen"
   goto exit