mg: allow to change the tab width

mg: allow to change the tab width

This makes the tab width customizable per-buffer.  The new function
`set-tab-width' changes it for the current buffer or the default value
for new buffers if called with a prefix argument (or from the startup
file.)

The default tab width is still 8 column.

Together with the newly resurrected no-tab-mode, allows to use mg for a
variety of programming languages and coding styles.

Note that it's not possible to call set-tab-width with auto-execute in
the startup file due to limitations in how auto-execute and the parser
work.

ok tb@