pgsql: Repair oversights in the mechanism used to store compiled plpgsql

Log Message:
-----------
Repair oversights in the mechanism used to store compiled plpgsql functions.
The original coding failed (tried to access deallocated memory) if there were
two active call sites (fn_extra pointers) for the same function and the
function definition was updated.  Also, if an update of a recursive function
was detected upon nested entry to the function, the existing compiled version
was summarily deallocated, resulting in crash upon return to the outer
instance.  Problem observed while studying a bug report from Sergiy
Vyshnevetskiy.

Bug does not exist before 8.1 since older versions just leaked the memory of
obsoleted compiled functions, rather than trying to reclaim it.

Modified Files:
--------------
    pgsql/src/pl/plpgsql/src:
        pl_comp.c (r1.109 -> r1.110)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_comp.c.diff?r1=1.109&r2=1.110)
        pl_handler.c (r1.35 -> r1.36)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_handler.c.diff?r1=1.35&r2=1.36)
        plpgsql.h (r1.83 -> r1.84)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/plpgsql.h.diff?r1=1.83&r2=1.84)