build: safe update of PATH on Windows (#3419)

NSIS has a default MAX_STR_LEN of 1024. If $ENV{PATH} is longer
the returned string is truncated to an empty string. Its then not
possible to distinguis between the variable not set or too long.
As a result the variable is set with the location where geth and/or
dev tools are installed. This may override any previous set values.

build/nsis.geth.nsi

@@ -17,8 +17,12 @@
 #
 # Requirements:
 # - NSIS, http://nsis.sourceforge.net/Main_Page
+# - NSIS Large Strings build, http://nsis.sourceforge.net/Special_Builds
 # - SFP, http://nsis.sourceforge.net/NSIS_Simple_Firewall_Plugin (put dll in NSIS\Plugins\x86-ansi)
 #
+# After intalling NSIS extra the NSIS Large Strings build zip and replace the makensis.exe and the
+# files found in Stub.
+#
 # based on: http://nsis.sourceforge.net/A_simple_installer_with_start_menu_shortcut_and_uninstaller
 #
 # TODO:
@@ -37,6 +41,7 @@ RequestExecutionLevel admin
 SetCompressor /SOLID lzma
 
 !include LogicLib.nsh
+!include PathUpdate.nsh
 !include EnvVarUpdate.nsh
 
 !macro VerifyUserIsAdmin