rename and clean unexec.c

[Dan Nicolaescu]

unexec.c is only used by MSDOS now.
It should be renamed unexecdos.c (or something like that) similar to all the 
other unexec files.

Also it needs a bit of a cleanup given that it's only used on MSDOS.


For example a few src/m/* files define NO_REMAP, but there's no need
for that, the only user is unexec.c
Maybe there are other #defines that can be removed in a similar fashion.

In fact almost all #ifdefs in unexec.c can be removed.