请按注释的说明，用C语言实现以下函数的功能。 /*Name:replace Function:to replace substring s1 in string source with substring s2 Parameters:source,string supposed to hold substring s1,substring to be replaced s2,substring to replace substring s1 flag,case-sensitive flag, 1,case-sensitive 0,non-case-sensitive Return values: number of substrings haved been replaced*/ (Supplementary:To be not complicated,assume that the length of the string after replaced is not greater than 1024 bytes.)