This read something like `o=!_validate_int`, and the flag modifier
reading kept the pointer after the `!`, so it created a long flag
called `_validate_int`, which meant it would not only error out form
```fish
argparse 'i=!_validate_int' 'o=!_validate_int' -- $argv
```
with "Long flag '_validate_int' already defined", but also set
$_flag_validate_int.
Fixes#5864.
This keeps all unknown options in $argv, so
```fish
argparse -i a/alpha -- -a banana -o val -w
```
results in $_flag_a set to banana, and $argv set to `-o val -w`.
This allows users to use multiple argparse passes, or to simply avoid
specifying all options e.g. in completions - `systemctl` has 46 of
them, most not having any effect on the completions.
Fixes#5367.
* Hoist `for` loop control var to enclosing scope
It should be possible to reference the last value assigned to a `for`
loop control var when the loop terminates. This makes it easier to detect
if we broke out of the loop among other things. This change makes fish
`for` loops behave like most other shells.
Fixes#1935
* Remove redundant line
While updating the `history` function to use `argparse` I realized it is
useful to define an option that can be used in three ways. First by
using the short flag; e.g., `-n NNN`. Second by using the long flag;
e.g., `--max NNN`. Third, as an implicit int flag; e.g., `-NNN`. This
use case is now supported by a spec of the form `n#max`.
This implements support for numeric flags without an associated short or
long flag name. This pattern is used by many commands. For example `head
-3 /a/file` to emit the first three lines of the file.
Fixes#4214
