strings: Remove NONNUL_ALL from NULL-taking functions.

vim_strup() is only used in a couple places, with no NULLs possible, so
keep the attribute and remove the NULL check.
api: Handle NULs and newlines in buffer_*_line.
api: Add tests for when not to handle NULs.
Merge pull request #1341 from splinterofchaos/api-nul

Api: Improve Nul handling
Merge pull request #1589 from splinterofchaos/fix-const-atter

strings: Remove NONNUL_ALL from NULL-taking functions.