* Unvectorize xstrlcpy

On all libcs I could find, memcpy does a better job than we can.

* Rename xstrlcpy to xstrsncpy to avoid confusion

The semantics of our xstrlcpy differed slightly from strlcpy; we return
the number of copied characters; strlcpy returns the strlen(src).
strscpy from Linux is similar except it returns the number of copied
characters sans the NUL byte, so this is named strsncpy to be different.

See https://no-color.org