loaders: PNM: Add support for binary G8.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
loaders: pnm: Avoid fgetc() in inner loops.

The fgetc() is guarded by lock to be thread safe which slows things down
more than hundred times.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>
spiv: Map the actions better to the F keys.

Now action triggered by F1 has cmdline switch -1 etc. and the -0 is
mapped to action 10 and F10.

Signed-off-by: Cyril Hrubis <metan@ucw.cz>