"Fill", as a new main-toolbar tool, was not recording snapshots
of the image for the "Undo" tool. Mended.
However, also updated the tool so that it doesn't _bother_ recording
into the undo buffer if the fill action is a no-op (e.g., clicking
the same spot a second time, or otherwise attempting to fill an
area with a color that's identical to what's already on the canvas).
Replace the somewhat-recently-added "nothing" item in the tool bar
with the "Fill" tool, which has been converted back from a Magic tool
to a regular tool.
Also, change bash "==" to sh "=" in "if [ ... ]" tests in Makefile.
Also, link to math library (via "-lm") to make sure "max()" is available to
magic tools.
Also, add missing mention of "--newcolorslast" and "--newcolorsfirst" to manpage.
