fix mixed tab/space

Hi there,

Most of the file use spaces for tabs, with 1 tab = 4 spaces, however there are a lot of tabs mixed in, with inconsistent size 2, 4 or 8.
It makes reading and modifying gdbinit difficult (for instance github diffs has a tab size of 8) so fixing it by sticking to the existing spaces convention.
Nothing else than tabs/spaces has been modified.

Cheers