Clarify internal_catch etc.

Clarify internal_catch etc.

The recent change to internal_catch and friends relied on some
confusion I introduced to the code in 2013.  Attempt to fix
the confusion by clarifying the code instead.  This saves an
instruction and a load dependency in the typical case.
* src/eval.c (internal_catch, internal_condition_case)
(internal_condition_case_1, internal_condition_case_2)
(internal_condition_case_n): Undo the previous change.  Instead,
use use ‘c’ rather than ‘handlerlist’ in the typical case.
Also, use ‘eassert’ rather than ‘clobbered_eassert’ when possible.