[cleanup patch] clean up AUTOLINEAR number input

[neroden]

Another thing I ran into while preparing the codebase for templated min/max.  The code for the AUTOLINEAR mode in gui_numberinput.cc was slightly confused about types (and also inconsistent between the decrement and the increment modes).  The PROGRESS mode was also confused about types and the integer promotion rules.

These have been fixed to behave in a consistent manner; patch attached.