Turn 'rem' comments into a real usage message in PCbuild/build.bat

Also fixes error in 'kill' target (already fixed in 2.7, somehow the fix
didn't make it to this branch).

PCbuild/build.bat

 @echo off
-rem A batch program to build or rebuild a particular configuration,
-rem just for convenience.
-
-rem Arguments:
-rem  -c  Set the configuration (default: Release)
-rem  -p  Set the platform (x64 or Win32, default: Win32)
-rem  -r  Target Rebuild instead of Build
-rem  -t  Set the target manually (Build, Rebuild, Clean, or CleanAll)
-rem  -d  Set the configuration to Debug
-rem  -e  Pull in external libraries using get_externals.bat
-rem  -m  Enable parallel build (enabled by default)
-rem  -M  Disable parallel build
-rem  -v  Increased output messages
-rem  -k  Attempt to kill any running Pythons before building (usually unnecessary)
+goto Run
+:Usage
+echo.%~nx0 [flags and arguments] [quoted MSBuild options]
+echo.
+echo.Build CPython from the command line.  Requires the appropriate
+echo.version(s) of Microsoft Visual Studio to be installed (see readme.txt).
+echo.Also requires Subversion (svn.exe) to be on PATH if the '-e' flag is
+echo.given.
+echo.
+echo.After the flags recognized by this script, up to 9 arguments to be passed
+echo.directly to MSBuild may be passed.  If the argument contains an '=', the
+echo.entire argument must be quoted (e.g. `%~nx0 "/p:PlatformToolset=v100"`)
+echo.
+echo.Available flags:
+echo.  -h  Display this help message
+echo.  -V  Display version information for the current build
+echo.  -r  Target Rebuild instead of Build
+echo.  -d  Set the configuration to Debug
+echo.  -e  Build external libraries fetched by get_externals.bat
+echo.  -m  Enable parallel build (enabled by default)
+echo.  -M  Disable parallel build
+echo.  -v  Increased output messages
+echo.  -k  Attempt to kill any running Pythons before building (usually done
+echo.      automatically by the pythoncore project)
+echo.
+echo.Available arguments:
+echo.  -c Release ^| Debug ^| PGInstrument ^| PGUpdate
+echo.     Set the configuration (default: Release)
+echo.  -p x64 ^| Win32
+echo.     Set the platform (default: Win32)
+echo.  -t Build ^| Rebuild ^| Clean ^| CleanAll
+echo.     Set the target manually
+exit /b 127
 
+:Run
 setlocal
 set platf=Win32
 set vs_platf=x86
@@ -25,6 +45,7 @@ set verbose=/nologo /v:m
 set kill=
 
 :CheckOpts
+if "%~1"=="-h" goto Usage
 if "%~1"=="-c" (set conf=%2) & shift & shift & goto CheckOpts
 if "%~1"=="-p" (set platf=%2) & shift & shift & goto CheckOpts
 if "%~1"=="-r" (set target=Rebuild) & shift & goto CheckOpts
@@ -43,7 +64,7 @@ rem Setup the environment
 call "%dir%env.bat" %vs_platf% >nul
 
 if "%kill%"=="true" (
-    msbuild /v:m /nologo /target:KillPython "%pcbuild%\pythoncore.vcxproj" /p:Configuration=%conf% /p:Platform=%platf% /p:KillPython=true
+    msbuild /v:m /nologo /target:KillPython "%dir%\pythoncore.vcxproj" /p:Configuration=%conf% /p:Platform=%platf% /p:KillPython=true
 )
 
 rem Call on MSBuild to do the work, echo the command.