Rename more files to accomodate 8+3 file systems, and adjust the

Rename more files to accomodate 8+3 file systems, and adjust the
vocabulary at the same time: AM_CONDITIONAL defines "conditionals",
and we call "condition" a conjunction of "conditionals".
* lib/am/Conditional.pm: Rename to lib/am/Condition.pm.
* lib/am/tests/Conditional.pl: Rename to lib/am/tests/Condition.pl.
* lib/am/ConditionalSet.pm: Rename to lib/am/DisjConditions.pm.
* lib/am/tests/ConditionalSet.pl: Rename to
lib/am/tests/DisjConditions.pl
* lib/am/Conditional.pm (condition_negate): Rename to ...
* lib/am/Condition.pm (conditional_negate): ... this.
* automake.in: Adjust references to Condition and DisjConditions.
(check_ambiguous_conditional, conditional_ambiguous_p): Rename to ...
(check_ambiguous_condition, condition_ambiguous_p): ... these.