resurrect mg' no-tab-mode

resurrect mg' no-tab-mode

It's a mode that makes mg insert spaces up to the next tab stop upon
pressing TAB, along with the various tweaks needed in other places so
for e.g. auto-indent-mode also uses spaces.

This is not just an unifdef NOTAB: even under no-tab-mode mg should
consider literal TAB characters wide up to the next tab stop, while the
hidden code considered hard tabs to be just control character (i.e. ^I)
with width of two columns.  I'm also introducing the helper function
doindent() in utils.c to de-obfuscate the insertion of tabs/spaces until
the given column.

ok tb@