Remove ancient code for FreeBSD 2.x compatibility.

Reviewed by: brian, freebsd-net
Don't calculate the tag until we know that we're going to make a new menu
item entry. Also join simple NULL assignments into a single line.
Oops, r252833 was not supposed to touch this file. Back-out and recommit
this file with the rest of the files it was supposed to go with.
Add support for processing add-on modules from /usr/local/libexec/bsdconfig
(this is designed to allow new modules to be installed via ports/packages).

To prevent conflict with itself (sysutils/bsdconfig) as a port (which
installs its base modules to the above directory, it was long-ago decided
that so-called `base' modules would look different than now-defined `add-on'
modules. The structure of the contents for each is the same, but the naming
convention for the module directory must be different.

Base modules are named `[0-9][0-9][0-9].*' to allow SysV-style organization
while add-on modules must avoid this naming style and are simply listed in
alphabetical order by their module directory.

For example, a hypothetical port named `bsdconfig-jails' could install
/usr/local/libexec/bsdconfig/jails and provide `bsdconfig jails' as well as
a new menu entry in the main-menu.

Add-on modules are listed in the main-menu (when bsdconfig is executed with-
out arguments) below a separator after the last base-module.

In `bsdconfig -h' output, add-on modules are listed right alongside base
modules (sorted alphabetically in columnar fashion; left-to-right).

If a base module declares a keyword used by an add-on module, the base
module will always win when given `bsdconfig keyword' syntax.

Add-on modules should avoid declaring any keyword found in `script.subr' as
a reserved-word (`Resword') since bsdconfig also supports `bsdconfig resword'
as a fall-back if no keyword is found to be declared by any module.
Fix the build with gcc.

Gcc outputs pre-UAL asm and expects the ldcl instruction with a condition
in the form ldc<c>l, where the code produces the instruction in the UAL
form ldcl<c>. Work around this by checking if we are using clang or gcc and
adjusting the instruction.

While here correct the cmp instruction's value to include the # before the
immediate value.