patch 8.0.1451: difficult to set the python home directories properly

Problem:    It is difficult to set the python home directory properly for
            Python 2.7 and 3.5 since both use $PYTHONHOME.
Solution:   Add the 'pythonhome' and 'pythonthreehome' options. (Kazuki
            Sakamoto, closes #1266)
diff --git a/runtime/doc/options.txt b/runtime/doc/options.txt
index f933083..75e4f97 100644
--- a/runtime/doc/options.txt
+++ b/runtime/doc/options.txt
@@ -5967,6 +5967,20 @@
 	This option cannot be set from a |modeline| or in the |sandbox|, for
 	security reasons.
 
+						*'pythonhome'*
+'pythonhome'		string	(default "")
+			global
+			{not in Vi}
+			{only available when compiled with the |+python/dyn|
+			feature}
+	Specifies the name of the Python 2.x home directory. When 'pythonhome'
+	and the PYTHONHOME environment variable are not set, PYTHON_HOME,
+	which was specified at compile time, will be used for the Python 2.x
+	home directory.
+	Environment variables are expanded |:set_env|.
+	This option cannot be set from a |modeline| or in the |sandbox|, for
+	security reasons.
+
 						*'pythonthreedll'*
 'pythonthreedll'	string	(default depends on the build)
 			global
@@ -5979,6 +5993,20 @@
 	This option cannot be set from a |modeline| or in the |sandbox|, for
 	security reasons.
 
+						*'pythonthreehome'*
+'pythonthreehome'	string	(default "")
+			global
+			{not in Vi}
+			{only available when compiled with the |+python3/dyn|
+			feature}
+	Specifies the name of the Python 3 home directory. When
+	'pythonthreehome' and the PYTHONHOME environment variable are not set,
+	PYTHON3_HOME, which was specified at compile time, will be used for
+	the Python 3 home directory.
+	Environment variables are expanded |:set_env|.
+	This option cannot be set from a |modeline| or in the |sandbox|, for
+	security reasons.
+
 						*'pyxversion'* *'pyx'*
 'pyxversion' 'pyx'	number	(default depends on the build)
 			global
diff --git a/runtime/doc/quickref.txt b/runtime/doc/quickref.txt
index 06ae663..516a1da 100644
--- a/runtime/doc/quickref.txt
+++ b/runtime/doc/quickref.txt
@@ -839,7 +839,9 @@
 'prompt'	  'prompt'  enable prompt in Ex mode
 'pumheight'	  'ph'	    maximum height of the popup menu
 'pythondll'		    name of the Python 2 dynamic library
+'pythonhome'		    name of the Python 2 home directory
 'pythonthreedll'	    name of the Python 3 dynamic library
+'pythonthreehome'	    name of the Python 3 home directory
 'pyxversion'	  'pyx'	    Python version used for pyx* commands
 'quoteescape'	  'qe'	    escape characters used in a string
 'readonly'	  'ro'	    disallow writing the buffer