[gdb/python] Warn and ignore ineffective python settings

[gdb/python] Warn and ignore ineffective python settings

Configuration flags "python dont-write-bytecode" and
"python ignore-environment" have effect only at Python initialization.

For instance, setting "python dont-write-bytecode" here has no effect:
...
$ gdb -q
(gdb) show python dont-write-bytecode
Python's dont-write-bytecode setting is auto (currently off).
(gdb) python import sys
(gdb) python print (sys.dont_write_bytecode)
False
(gdb) set python dont-write-bytecode on
(gdb) python print (sys.dont_write_bytecode)
False
...

This is not clear in the code: we set Py_DontWriteBytecodeFlag and
Py_IgnoreEnvironmentFlag in set_python_ignore_environment and
set_python_dont_write_bytecode.  Fix this by moving the setting of those
variables to py_initialization.

Furthermore, this is not clear to the user: after Python initialization, the
user can still modify the configuration flags, and observe the changed setting:
...
$ gdb -q
(gdb) show python ignore-environment
Python's ignore-environment setting is off.
(gdb) set python ignore-environment on
(gdb) show python ignore-environment
Python's ignore-environment setting is on.
(gdb)
...

Fix this by emitting a warning when trying to set these configuration flags
after Python initialization:
...
$ gdb -q
(gdb) set python ignore-environment on
warning: Setting python ignore-environment after Python initialization has \
  no effect, try setting this during early initialization
(gdb) set python dont-write-bytecode on
warning: Setting python dont-write-bytecode after Python initialization has \
  no effect, try setting this during early initialization, or try setting \
  sys.dont_write_bytecode
...
and by keeping the values constant after Python initialization.

Since the auto setting for python dont-write-bytecode depends on the current
value of environment variable PYTHONDONTWRITEBYTECODE, we simply avoid it
after Python initialization:
...
$ gdb -q -batch \
    -eiex "show python dont-write-bytecode" \
    -iex "show python dont-write-bytecode"
Python's dont-write-bytecode setting is auto (currently off).
Python's dont-write-bytecode setting is off.
...

Tested on aarch64-linux.

Approved-By: Tom Tromey <tom@tromey.com>
PR python/32388
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=32388