Add --help and --version

--help is alias of -h
--version is alias of -V

xxhsum.1.md

@@ -33,7 +33,7 @@ OPTIONS
 * `-c`, `--check`:
   Read xxHash sums from the <FILE>s and check them
 
-* `-h`:
+* `-h`, `--help`:
   Display help and exit
 
 * `-H`<HASHTYPE>:
@@ -44,7 +44,7 @@ OPTIONS
   Set output hexadecimal checksum value as little endian convention.
   By default, value is displayed as big endian
 
-* `-V`:
+* `-V`, `--version`:
   Display xxhsum version
 
 **The following four options are useful only when verifying checksums (`-c`)**

xxhsum.c

@@ -1297,6 +1297,8 @@ int main(int argc, const char** argv)
         if (!strcmp(argument, "--status")) { statusOnly = 1; continue; }
         if (!strcmp(argument, "--quiet")) { quiet = 1; continue; }
         if (!strcmp(argument, "--warn")) { warn = 1; continue; }
+        if (!strcmp(argument, "--help")) { return usage_advanced(exename); }
+        if (!strcmp(argument, "--version")) { DISPLAY(WELCOME_MESSAGE); return 0; }
 
         if (*argument!='-')
         {