cmake_parse_arguments: Restore capture of value after repeated keyword

When a single-value keyword is repeated, and the first instance is missing a value, it prevents the value from the second instance from being stored in a variable. This was a regression introduced by !9748 (merged). That change also didn't create a variable if the keyword was given but without a value. The purpose of the change was to always define a variable if a keyword was given. Lastly, that change didn't protect the CMP0174 logic to make it only apply to the PARSE_ARGV form.

The first two of the above problems are fixed here by tracking the keywords given instead of checking which keywords were missing values. The third problem is also fixed here, being tightly coupled to the same logic as the first two problems.

Fixes: #26397 (closed)
Backport: release