fix(strvec_{posix,str}cmp): use unsigned char to be consistent with strcmp

[akinomyoga]

The present code tries to reduce the call to strcmp by comparing the first character of the strings inline, but the comparison is made by char (which is signed in a typical environment). This is inconsistent with strcmp, which compares strings using unsigned char.

I'm not sure if I need to show an actual affected case, but it is hard to create a natural case. I'm not sure if this can be robustly reproducible in other systems, but the following demonstrates the issue in my system (Linux, with strcoll(3) from glibc).

$ mkdir t
$ cd t
$ LANG=C
$ touch {,a}{$'\1',$'\xA9'}.txt
$ LANG=en_US.UTF-8
$ printf'%s\n'*.txt | cat -va^A.txtaM-).txtM-).txt^A.txt

\x01 and \xA9 are shown by ^A and M-) in the cat -v output. Here, we can observe that, when \x01 and \xA9 are the second characters of the filenames, \x01 comes first. However, when they are the first characters of the filenames, \xA9 comes first because it is a negative number (-87) in char.

[akinomyoga]

Another possibility is to prepare a function for the common part:

staticintstrvec_truestrcmp (char**s1, char**s2)
{
intresult;
if ((result= (unsigned char)**s1- (unsigned char)**s2) ==0)
result=strcmp (*s1, *s2);
returnresult;
}

and make strvec_posixcmp and strvec_strcmp call strvec_truestrcmp.