[Bug] 'command readonly/export' exit on error

[Martijn Dekker]

The 'readonly' and 'export' commands exit the shell on error even if 
they are prefixed with 'command', which should stop that exit from 
happening.

Test cases:
$ mksh -o posix -c 'readonly v; command export v=foo || echo ok'
mksh: read-only: v
$ mksh -o posix -c 'readonly v; command readonly v=foo || echo ok'
mksh: read-only: v

('ok' should be printed in both cases)

Thanks,

- M.