lib/string.c: use the name "C-string" in comments

For strncpy() and friends the source string may or may not have an actual
NUL character at the end.  The documentation is confusing in this because
it specifically mentions that you are passing a "NUL-terminated" string.
Wikipedia says that "C-string" is an alternative name we can use instead.

http://en.wikipedia.org/wiki/Null-terminated_string

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>