### Before
Case 1:
```xsh
xonsh --no-rc --no-env -DCOLOR_OUTPUT 0 # space between -DVAR and VALUE
```
```xsh
Traceback (most recent call last):
File "/Users/pc/.local/xonsh-env/lib/python3.12/site-packages/xonsh/main.py", line 478, in main
args = premain(argv)
^^^^^^^^^^^^^
File "/Users/pc/.local/xonsh-env/lib/python3.12/site-packages/xonsh/main.py", line 420, in premain
env.update([x.split("=", 1) for x in args.defines])
File "<frozen _collections_abc>", line 987, in update
ValueError: not enough values to unpack (expected 2, got 1)
Xonsh encountered an issue during launch
```
Case 2:
```xsh
xonsh --no-rc --no-env -DXONSH_HISTORY_BACKEND=dummy
history info
# json # EXPECTED: dummy
```
### After
Case 1:
```xsh
xonsh --no-rc --no-env -DCOLOR_OUTPUT 0
# Wrong format for -DCOLOR_OUTPUT argument. Use -DVAR=VAL form.
```
Case 2:
```xsh
xonsh --no-rc --no-env -DXONSH_HISTORY_BACKEND=dummy
history info
# dummy
```
Closes#5395
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