Ugly. A pile of new xxxFlags() functions, to communicate to the parser

that 'yield' is a keyword.  This doesn't help test_generators at all!  I
don't know why not.  These things do work now (and didn't before this
patch):

1. "from __future__ import generators" now works in a native shell.

2. Similarly "python -i xxx.py" now has generators enabled in the
   shell if xxx.py had them enabled.

3. This program (which was my doctest proxy) works fine:

from __future__ import generators

source = """\
def f():
    yield 1
"""

exec compile(source, "", "single") in globals()
print type(f())

1 files changed, 22 insertions, 5 deletions

diff --git a/Parser/parsetok.c b/Parser/parsetok.c
index b68fbd7..6017e5f 100644
--- a/Parser/parsetok.c
+++ b/Parser/parsetok.c
@@ -13,13 +13,19 @@ int Py_TabcheckFlag;
 
 
 /* Forward */
-static node *parsetok(struct tok_state *, grammar *, int, perrdetail *);
+static node *parsetok(struct tok_state *, grammar *, int, perrdetail *, int);
 
 /* Parse input coming from a string.  Return error code, print some errors. */
-
 node *
 PyParser_ParseString(char *s, grammar *g, int start, perrdetail *err_ret)
 {
+	return PyParser_ParseStringFlags(s, g, start, err_ret, 0);
+}
+
+node *
+PyParser_ParseStringFlags(char *s, grammar *g, int start,
+		          perrdetail *err_ret, int flags)
+{
 	struct tok_state *tok;
 
 	err_ret->error = E_OK;
@@ -42,7 +48,7 @@ PyParser_ParseString(char *s, grammar *g, int start, perrdetail *err_ret)
 			tok->alterror++;
 	}
 
-	return parsetok(tok, g, start, err_ret);
+	return parsetok(tok, g, start, err_ret, flags);
 }
 
 
@@ -52,6 +58,14 @@ node *
 PyParser_ParseFile(FILE *fp, char *filename, grammar *g, int start,
 		   char *ps1, char *ps2, perrdetail *err_ret)
 {
+	return PyParser_ParseFileFlags(fp, filename, g, start, ps1, ps2,
+				       err_ret, 0);
+}
+
+node *
+PyParser_ParseFileFlags(FILE *fp, char *filename, grammar *g, int start,
+			char *ps1, char *ps2, perrdetail *err_ret, int flags)
+{
 	struct tok_state *tok;
 
 	err_ret->error = E_OK;
@@ -72,14 +86,15 @@ PyParser_ParseFile(FILE *fp, char *filename, grammar *g, int start,
 	}
 
 
-	return parsetok(tok, g, start, err_ret);
+	return parsetok(tok, g, start, err_ret, flags);
 }
 
 /* Parse input coming from the given tokenizer structure.
    Return error code. */
 
 static node *
-parsetok(struct tok_state *tok, grammar *g, int start, perrdetail *err_ret)
+parsetok(struct tok_state *tok, grammar *g, int start, perrdetail *err_ret,
+	 int flags)
 {
 	parser_state *ps;
 	node *n;
@@ -90,6 +105,8 @@ parsetok(struct tok_state *tok, grammar *g, int start, perrdetail *err_ret)
 		err_ret->error = E_NOMEM;
 		return NULL;
 	}
+	if (flags & PyPARSE_YIELD_IS_KEYWORD)
+		ps->p_generators = 1;
 
 	for (;;) {
 		char *a, *b;