tools include: Adopt strstarts() from the kernel

Replacing prefixcmp(), same purpose, inverted result, so standardize on
the kernel variant, to reduce silly differences among tools/ and the
kernel sources, making it easier for people to work in both codebases.

And then doing:

	if (strstarts(option, "no-"))

Looks clearer than doing:

	if (!prefixcmp(option, "no-"))

To figure out if option starts witn "no-".

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-kaei42gi7lpa8subwtv7eug8@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
diff --git a/tools/perf/builtin-help.c b/tools/perf/builtin-help.c
index 530a7f2..dbe4e41 100644
--- a/tools/perf/builtin-help.c
+++ b/tools/perf/builtin-help.c
@@ -90,7 +90,7 @@ static int check_emacsclient_version(void)
 	 */
 	finish_command(&ec_process);
 
-	if (prefixcmp(buffer.buf, "emacsclient")) {
+	if (!strstarts(buffer.buf, "emacsclient")) {
 		fprintf(stderr, "Failed to parse emacsclient version.\n");
 		goto out;
 	}
@@ -283,7 +283,7 @@ static int perf_help_config(const char *var, const char *value, void *cb)
 		add_man_viewer(value);
 		return 0;
 	}
-	if (!prefixcmp(var, "man."))
+	if (!strstarts(var, "man."))
 		return add_man_viewer_info(var, value);
 
 	return 0;
@@ -313,7 +313,7 @@ static const char *cmd_to_page(const char *perf_cmd)
 
 	if (!perf_cmd)
 		return "perf";
-	else if (!prefixcmp(perf_cmd, "perf"))
+	else if (!strstarts(perf_cmd, "perf"))
 		return perf_cmd;
 
 	return asprintf(&s, "perf-%s", perf_cmd) < 0 ? NULL : s;