dist: Add a startup shell script and manpage

Pretty basic shell script for finding a port and launching it
automatically.  It's nothing overly fancy, but shell portability is
strived for, since it should be capable of launching no matter what
operating system is in use, it should not use anything more advanced
than is available in Bourn shell/POSIX (no ksh/bash extensions).
dist: Add boom/doom in the exec script's PORTS list

These are generic names used by at least Debian to allow users to
specify their favored port with the package manager.  They can be used
as another mode of preference with the script.
dist: Switch *.desktop files to use the freedoom command
Makefile: Add install targets

These targets should help both users installing Freedoom manually and
distribution packagers (especially the individual targets for each
IWAD).