Library: Make the string functions match the prototypes in stdlib.h

Library: Make the string functions match the prototypes in stdlib.h

Rename single-argument ltoa and ultoa to _ltoa and _ultoa.

Rename non-standard ltostr and ultostr to __ltostr and __ultostr, change
prototypes in stdlib.h to match expected arguments.

New implementations of ltoa, ultoa based on __ltostr, __ultostr.

Change vfprintf to call __ltostr and __ultostr.

Change _itoa to call _ltoa.

Now we have:

_ltoa, _ultoa, _itoa: Take a single argument (value).

__ltostr, __ultostr: Take two arguments (value, radix).

ltoa, ultoa: Take three arguments (value, buffer, radix). Calls either
__ltostr or __ultostr and copies the result into the user-provided
buffer.