Fix -Wextra Warnings

Cast size_t to long in long-size_t comparisons

````
In file included from ../include/libut.h:7:
../include/utstring.h:167:14: warning: comparison of integers of different signs: 'long' and 'size_t' (aka 'unsigned long') [-Wsign-compare]
    while (i < P_NeedleLen)
           ~ ^ ~~~~~~~~~~~
../include/utstring.h:175:15: warning: comparison of integers of different signs: 'long' and 'size_t' (aka 'unsigned long') [-Wsign-compare]
        if (i < P_NeedleLen)
            ~ ^ ~~~~~~~~~~~
../include/utstring.h:209:20: warning: comparison of integers of different signs: 'long' and 'size_t' (aka 'unsigned long') [-Wsign-compare]
        while ( (j < P_NeedleLen) && (P_Needle[i] != P_Needle[j]) )
                 ~ ^ ~~~~~~~~~~~
../include/utstring.h:324:25: warning: comparison of integers of different signs: 'long' and 'size_t' (aka 'unsigned long') [-Wsign-compare]
    if ( (V_HaystackLen >= P_NeedleLen) && (P_NeedleLen > 0) )
          ~~~~~~~~~~~~~ ^  ~~~~~~~~~~~
../include/utstring.h:370:25: warning: comparison of integers of different signs: 'long' and 'size_t' (aka 'unsigned long') [-Wsign-compare]
    if ( (V_HaystackLen >= P_NeedleLen) && (P_NeedleLen > 0) )
          ~~~~~~~~~~~~~ ^  ~~~~~~~~~~~
5 warnings generated.
````