Updated BerkeleyDB version tests

reduce library link tries.

not yet used

Do not use test -a/-o.  Use shell features like &&, || or case...esac.
Use grep instead of egrep (but not the not-yet-portable Posix variant grep -E).
Based on a patch from Paul Eggert.

the passed in help string. (Append the valid args to the help string...)

Use AC_CHECK_TYPE(S) instead of old macros

of overriding individual settings.
The --disable variant will disable all backends/overlays that are
not explicitly enabled.  Previously it had no effect.
Also shorten a few --enable help messages that were > 80 characters.

header doesn't precisely match the return of db_version().