dehacked: Allow override of string replacements.

If loading two dehacked patches and both replace the same string,
the second replacement should override the first. Change the API
function DEH_AddStringReplacement so that the from_text and to_text
are implicitly duplicated, and we can free to_text and replace
it later if we subsequently change it to something else.
Fix dehacked patch loading order.

The order in which we load dehacked patches is important. Change the
order so that IWAD dehacked patches are loaded before any others, and
so if, for example, we're playing with Freedoom, the Freedoom string
replacements can be overridden by those from extra mods we're playing
with.

As part of this, ditch DEH_Init() and use DEH_ParseCommandLine()
instead to handle the -deh option. Remove the DEH_Init() message
from startup and show messages about dehacked patches that we load
with the WAD files that we load.