tools lib string: Adopt prefixcmp() from perf and subcmd

Both had copies originating from git.git, move those to
tools/lib/string.c, getting both tools/lib/subcmd/ and tools/perf/ to
use it.

Cc: Adrian Hunter <[e-mail]>
Cc: David Ahern <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Josh Poimboeuf <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Wang Nan <[e-mail]>
Link: http://lkml.kernel.org/n/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/include/linux/string.h

···
 
 char *str_error_r(int errnum, char *buf, size_t buflen);
 
+int prefixcmp(const char *str, const char *prefix);
+
 #endif /* _LINUX_STRING_H_ */

tools/lib/string.c

···
 	}
 	return ret;
 }
+
+int prefixcmp(const char *str, const char *prefix)
+{
+	for (; ; str++, prefix++)
+		if (!*prefix)
+			return 0;
+		else if (*str != *prefix)
+			return (unsigned char)*prefix - (unsigned char)*str;
+}

tools/lib/subcmd/help.c

···
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
+#include <linux/string.h>
 #include <termios.h>
 #include <sys/ioctl.h>
 #include <sys/types.h>

tools/lib/subcmd/parse-options.c

···
 #include <linux/compiler.h>
+#include <linux/string.h>
 #include <linux/types.h>
 #include <stdio.h>
 #include <stdlib.h>

tools/lib/subcmd/subcmd-util.h

···
 	free(tmp);
 }
 
-static inline int prefixcmp(const char *str, const char *prefix)
-{
-	for (; ; str++, prefix++)
-		if (!*prefix)
-			return 0;
-		else if (*str != *prefix)
-			return (unsigned char)*prefix - (unsigned char)*str;
-}
-
 #endif /* __SUBCMD_UTIL_H */

tools/perf/util/strbuf.c

···
 #include <linux/kernel.h>
 #include <errno.h>
 
-int prefixcmp(const char *str, const char *prefix)
-{
-	for (; ; str++, prefix++)
-		if (!*prefix)
-			return 0;
-		else if (*str != *prefix)
-			return (unsigned char)*prefix - (unsigned char)*str;
-}
-
 /*
  * Used as the default ->buf value, so that people can always assume
  * buf is non NULL and ->buf is NUL terminated even for a freshly

tools/perf/util/util.h

···
 
 void set_warning_routine(void (*routine)(const char *err, va_list params));
 
-int prefixcmp(const char *str, const char *prefix);
-
 static inline void *zalloc(size_t size)
 {
 	return calloc(1, size);