Bug 4610: cleanup of BerkleyDB related checks

Bug 4610: cleanup of BerkleyDB related checks

Most of the logic seems to be hangovers from when session helper was
using the BerkleyDB v1.85 compatibility interface. Some of it is
possibly still necessary for the time_quota helper, but that helper has
not been using it so far and needs an upgrade to match what happened to
session helper.

Changes:

* The helpers needing -ldb will not be built unless the library and
headers are available. So we can drop the Makefile LIB_DB substitutions
and always just link -ldb explicitly to these helpers.

NP: Anyone who needs small minimal binaries, can build with the
--as-needed linker flag, or without these helpers. This change has no
effect on other helpers or the main squid binary.

* Since we no longer need to check if -ldb is necessary, we can drop the
configure.ac and acinclude logic detecting that.

* Remove unused AC_CHECK_DECL(dbopen, ...)
 - resolves one "FIXME"

* Fix the time_quota helper check to only scan db.h header file contents
if that file is existing, and if the db_185.h file is not being used
instead.

* Fix the session helper check to only try compiling with the db.h
header if that header actually exists.

* De-duplicate the library header file detection shared by configure.ac
and the helpers required.m4 files (after the above two changes).

* Remove unused DBLIB variable from configure.ac.