Fixes bug in strlower_libc_sb()

Hi Hackers,

While reviewing Jeff's patch [1], I found this bug in strlower_libc_sb():

```
static size_t
strlower_libc_sb(char *dest, size_t destsize, const char *src, ssize_t
srclen,
pg_locale_t locale)
{
if (srclen < 0)
srclen = strlen(src);

if (srclen + 1 <= destsize)
{
locale_t loc = locale->lt;
char *p;

if (srclen + 1 > destsize) <== This check will never be true
return srclen;
```

So I removed the useless check in the patch.

I also noticed strlower_libc_sb’s behavior is different
from unicode_strlower(), where the function comment says:

```
* Result string is stored in dst, truncating if larger than dstsize. If
* dstsize is greater than the result length, dst will be NUL-terminated;
* otherwise not.
```

And Jeff’s new code in [1] matches the description. The behavior is like
trying the best to copy as many chars as possible, if not enough space in
dest, then NULL-terminator can be omitted. But the current behavior
of strlower_libc_sb that only copies data to dest when dest has enough
space for src and NULL-terminator.

So I updated strlower_libc_sb(), strtitle_libc_sb() and strupper_libc_sb()
to comply with the behavior in this patch as well. “Make check” passed from
my side, so the patch doesn't seem to break anything.

[1]
https://www.postgresql.org/message-id/450ceb6260cad30d7afdf155d991a9caafee7c0d.camel%40j-davis.com

Best regards,
--
Chao Li (Evan)
HighGo Software Co., Ltd.
https://www.highgo.com/