Increasing the default kern.argmax

[Thomas Klausner <wiz%NetBSD.org@localhost>]

Hi!

Recently, some pkgsrc packages started to need command lines longer
than 256kB. (grafana, for its massive distfiles, some rust crates with
too long linker lines.)

This is currently only a problem for NetBSD, because Linux and macOS
(not sure about others) have a default limit of 1MB.

I think we should follow along and increase the default to 1MB too.

Comments?
 Thomas