perf subcmd: Create subcmd library

Move the subcommand-related files from perf to a new library named
libsubcmd.a.

Since we're moving files anyway, go ahead and rename 'exec_cmd.*' to
'exec-cmd.*' to be consistent with the naming of all the other files.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/c0a838d4c878ab17fee50998811612b2281355c1.1450193761.git.jpoimboe@redhat.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/parse-events.c

@@ -4,9 +4,9 @@
 #include "../perf.h"
 #include "evlist.h"
 #include "evsel.h"
-#include "parse-options.h"
+#include <subcmd/parse-options.h>
 #include "parse-events.h"
-#include "exec_cmd.h"
+#include <subcmd/exec-cmd.h>
 #include "string.h"
 #include "symbol.h"
 #include "cache.h"