unbound: Move config.h to lib directory.

It was originally in contrib, and moved to usr.sbin in 6692aa840c1f; I always thought lib would make more sense but never got around to moving it.

Reviewed by: cy
Differential Revision: https://reviews.freebsd.org/D41504

Try harder to sanitize the environment before running configure.
Remove a workaround for older Unbound versions that used sbrk.

Approved by: re (gjb)

Ask make(1) which compiler to use rather than rely on whatever is in $PATH.

Clean up the libunbound build to avoid accidentally regenerating the
configuration lexer and parser during buildworld. Instead of being
included in the source as it is in the upstream distribution, the code is
now always generated (in ${.OBJDIR}) at build time.

PR: 190739
MFC after: 1 week

Regenerate the configure script before running it.
Set the default config file to /var/unbound/unbound.conf.

Approved by: re (blanket)

Due to a missing command-line argument, yacc regenerated the parser but
not its header file, resulting in a mismatch between the lexer and parser
and strange errors when reading the configuration file.

Approved by: re (blanket)

Set props and correct RCS ID tag.

Approved by: re (blanket)

Set the correct prefix and exec-prefix.

Approved by: re (blanket)

Two helper scripts for porting Unbound:

- freebsd-configure.sh runs ./configure with the correct parameters
and regenerates the lex and yacc code.

- freebsd-sources.pl untangles the upstream Makefile and generates
source lists for our Makefiles.

Approved by: re (blanket)