Change the return type of freelocale(3) to void.

Our version of this function currently returns an integer indicating
failure or success, whereas POSIX specifies that this function has no
return value. It returns void. Patch up the header, sources and man page
to use the right type. While there, use the opportunity to simplify the
body of this function.

Theoretically speaking, this change breaks the ABI of this function.
That said, I have yet to find any code that makes use of freelocale()'s
return value. I couldn't find any of it in the base system, nor did an
exp-run reveal any breakage caused by this change.

PR: 211394 (exp-run)