exec builtin doesn't take flags

[lilyball]

The exec builtin seems to support parsing flags (based on error messages if you try and provide one), but it doesn't actually define any flags. Bash's exec takes 3 flags, 2 of which are clearly useful and one of which is questionable. I think we should at least add support for the -l and -a flags.

• -l: If the first argument to exec is -l then this prefixes the zeroth arg with a dash as login does.
• -a name: This flag passes name as the zeroth arg.

exec -l foo is clearly just shorthand for exec -a -foo foo, but is admittedly convenient.

As it is, if I want to replace my fish shell with a command using a custom name I need to write something like exec bash -c 'exec -a foo bar' which is silly.

[floam]

Yeah, I'd like -a or an --argv0. As it is, I have to write small C programs to affect these.

[floam]

These could also be arguments for command.