POSIX just says that mbstate_t should be an "object type other than an
array type" that can hold the conversion state for converting between
(multi-byte) characters and wide characters.
Since no other information regarding the contents is given, this
apparently means that we should add whatever we need once we start
implementing that conversion.
wint_t is also not supposed to be defined by sys/types.h, but should
be defined in wchar.h instead. Since we require it for our definition of
btowc, let's move it to the correct place.
SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
