pgsql: Use heap_modify_tuple not SPI_modifytuple in pl/python triggers.

Use heap_modify_tuple not SPI_modifytuple in pl/python triggers.

The code here would need some change anyway given planned change in
SPI_modifytuple semantics, since this executes after we've exited the
SPI environment. But really it's better to just use heap_modify_tuple.

While at it, normalize use of SPI_fnumber: make error messages distinguish
no-such-column from can't-set-system-column, and remove test for deleted
column which is going to migrate into SPI_fnumber. The lack of a check
for system column names is actually a pre-existing bug here, and might
even qualify as a security bug except that we don't have any trusted
version of plpython.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/de4026c673f195cfdb7aa7cc87cc60e36963f094

Modified Files
--------------
src/pl/plpython/plpy_exec.c | 86 +++++++++++++++++++++------------------------
1 file changed, 41 insertions(+), 45 deletions(-)