PROBLEM: putting in a loader config file a line of the form

loader_conf_files="foo bar baz"

should cause loading the files listed, and then resume with the
remaining config files (from previous values of the variable).
Unfortunately, sometimes the line was ignored -- actually even
modifying the line in /boot/default/loader.conf sometimes doesn't work.

ANALYSIS: After much investigation, turned out to be a bug in the logic.
The existing code detected a new assignment by looking at the address
of the the variable containing the string. This only worked by pure
chance, i.e. if the new string is longer than the previous value
then the memory allocator may return a different address
to store the string hence triggering the detection.

SOLUTION: This commit contains a minimal change to fix the problem,
without altering too much the existing structure of the code.
However, as a step towards improving the quality and reliability of
this code, I have introduced a handful of one-line functions
(strget, strset, strfree, string= ) that could be used in dozens
of places in the existing code.

HOWEVER:
There is a much bigger problem here. Even though I am no Forth
expert (as most fellow src committers) I can tell that much of the
forth code (in support.4th at least) is in severe need of a
review/refactoring:

+ pieces of code are replicated multiple times instead of writing

functions (see e.g.  set_module_*);

+ a lot of stale code (e.g. "structure" definitions for

preloaded_files, kernel_module, pnp stuff) which is not used
or at least belongs elsewhere.
The code bload is extremely bad as the loader runs with very small
memory constraints, and we already hit the limit once (see

  http://svn.freebsd.org/viewvc/base?view=revision&revision=185132
Reducing the footprint of the forth files is critical.

+ two different styles of coding, one using pure stack functions

(maybe beautiful but surely highly unreadable), one using
high level mechanisms to give names to arguments and local
variables (which leads to readable code).

Note that this code is used by default by all FreeBSD installations,
so the fragility and the code bloat are extremely damaging.
I will try to work fixing the three items above, but if others have
time, please have a look at these issues.

MFC after: 4 weeks