lib/string.c: implement stpcpy for 4.14, 4.9, and 4.4

Dear stable kernel maintainers, Please consider applying the following patches to 4.14, 4.9, and 4.4 implementing stpcpy, which is required when building w/ clang-12 and newer. These are manual backports of patches already accepted into stable earlier today, but for branches which they do not cherry pick back cleanly, due to us not backporting commit 458a3bf82df4 ("lib/string: Add strscpy_pad() function") which landed in v5.2-rc1.

Thanks to Nathan Chancellor for taking the time to backport these. The 4.9 and 4.4 patches look the same to me (other than the base).

(We've been running them under CI to ensure our builds stay green. I would like to drop them.)