Found with pmd-cpd scanner. The new function could probably be named better.

This reverts commit 9f0b23f49e.

Firstly, a ~ match is supposed to match even if there's nothing left.

Also, when p doesn't match, we should not return immediately, but skip
to the next group.

Fixes #1507

Note: The latter part is untested code.

TODO: think of a better function name.

One redundant ix++ was removed because there's a return right after that.

add tolower and toupper to be processed by ucd-tools in WCTYPE_H_COMPAT_SHIM

some vendors don't provide conformance with c standard in bionic

See commits ae97fa16 and a178ea1b.