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/src/if_python.c b/src/if_python.c
index f9fc5e1..ab9c854 100644
--- a/src/if_python.c
+++ b/src/if_python.c
@@ -912,6 +912,8 @@
 }
 #endif
 
+static char *py_home_buf = NULL;
+
     static int
 Python_Init(void)
 {
@@ -929,10 +931,15 @@
 	}
 #endif
 
+	if (*p_pyhome != NUL)
+	{
+	    /* The string must not change later, make a copy in static memory. */
+	    py_home_buf = (char *)vim_strsave(p_pyhome);
+	    if (py_home_buf != NULL)
+		Py_SetPythonHome(py_home_buf);
+	}
 #ifdef PYTHON_HOME
-# ifdef DYNAMIC_PYTHON
-	if (mch_getenv((char_u *)"PYTHONHOME") == NULL)
-# endif
+	else if (mch_getenv((char_u *)"PYTHONHOME") == NULL)
 	    Py_SetPythonHome(PYTHON_HOME);
 #endif
 
diff --git a/src/if_python3.c b/src/if_python3.c
index 7aa5f03..02d9134 100644
--- a/src/if_python3.c
+++ b/src/if_python3.c
@@ -842,6 +842,8 @@
 }
 #endif
 
+static wchar_t *py_home_buf = NULL;
+
     static int
 Python3_Init(void)
 {
@@ -857,11 +859,18 @@
 
 	init_structs();
 
+	if (*p_py3home != NUL)
+	{
+	    size_t len = mbstowcs(NULL, (char *)p_py3home, 0) + 1;
 
+	    /* The string must not change later, make a copy in static memory. */
+	    py_home_buf = (wchar_t *)alloc(len * sizeof(wchar_t));
+	    if (py_home_buf != NULL && mbstowcs(
+			    py_home_buf, (char *)p_py3home, len) != (size_t)-1)
+		Py_SetPythonHome(py_home_buf);
+	}
 #ifdef PYTHON3_HOME
-# ifdef DYNAMIC_PYTHON3
-	if (mch_getenv((char_u *)"PYTHONHOME") == NULL)
-# endif
+	else if (mch_getenv((char_u *)"PYTHONHOME") == NULL)
 	    Py_SetPythonHome(PYTHON3_HOME);
 #endif
 
diff --git a/src/option.c b/src/option.c
index 08fb789..ce54369 100644
--- a/src/option.c
+++ b/src/option.c
@@ -2248,6 +2248,15 @@
 			    {(char_u *)NULL, (char_u *)0L}
 #endif
 			    SCRIPTID_INIT},
+    {"pythonthreehome", NULL,   P_STRING|P_EXPAND|P_VI_DEF|P_SECURE,
+#if defined(FEAT_PYTHON3)
+			    (char_u *)&p_py3home, PV_NONE,
+			    {(char_u *)"", (char_u *)0L}
+#else
+			    (char_u *)NULL, PV_NONE,
+			    {(char_u *)NULL, (char_u *)0L}
+#endif
+			    SCRIPTID_INIT},
     {"pythondll",   NULL,   P_STRING|P_EXPAND|P_VI_DEF|P_SECURE,
 #if defined(DYNAMIC_PYTHON)
 			    (char_u *)&p_pydll, PV_NONE,
@@ -2257,6 +2266,15 @@
 			    {(char_u *)NULL, (char_u *)0L}
 #endif
 			    SCRIPTID_INIT},
+    {"pythonhome",  NULL,   P_STRING|P_EXPAND|P_VI_DEF|P_SECURE,
+#if defined(FEAT_PYTHON)
+			    (char_u *)&p_pyhome, PV_NONE,
+			    {(char_u *)"", (char_u *)0L}
+#else
+			    (char_u *)NULL, PV_NONE,
+			    {(char_u *)NULL, (char_u *)0L}
+#endif
+			    SCRIPTID_INIT},
     {"pyxversion", "pyx",   P_NUM|P_VI_DEF|P_SECURE,
 #if defined(FEAT_PYTHON) || defined(FEAT_PYTHON3)
 			    (char_u *)&p_pyx, PV_NONE,
diff --git a/src/option.h b/src/option.h
index c9fa4d3..21be3a3 100644
--- a/src/option.h
+++ b/src/option.h
@@ -696,9 +696,15 @@
 #if defined(DYNAMIC_PYTHON3)
 EXTERN char_u	*p_py3dll;	/* 'pythonthreedll' */
 #endif
+#ifdef FEAT_PYTHON3
+EXTERN char_u	*p_py3home;	/* 'pythonthreehome' */
+#endif
 #if defined(DYNAMIC_PYTHON)
 EXTERN char_u	*p_pydll;	/* 'pythondll' */
 #endif
+#ifdef FEAT_PYTHON
+EXTERN char_u	*p_pyhome;	/* 'pythonhome' */
+#endif
 #if defined(FEAT_PYTHON) || defined(FEAT_PYTHON3)
 EXTERN long	p_pyx;		/* 'pyxversion' */
 #endif
diff --git a/src/version.c b/src/version.c
index ea5dbef..e01f2be 100644
--- a/src/version.c
+++ b/src/version.c
@@ -772,6 +772,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1451,
+/**/
     1450,
 /**/
     1449,