perf tools: Move help_unknown_cmd() to its own file

help_unknown_cmd() is quite perf-specific because it relies on some
perf_config*() functions.  Move it and its supporting functions out into
a separate file so that help.c can be moved to a library.

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/562d918bcaaf340c1ae3e47586b3f0ae33b9918b.1449965119.git.jpoimboe@redhat.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
diff --git a/tools/perf/util/Build b/tools/perf/util/Build
index 65fef59..99b3dae 100644
--- a/tools/perf/util/Build
+++ b/tools/perf/util/Build
@@ -87,6 +87,7 @@
 libperf-y += parse-branch-options.o
 libperf-y += parse-regs-options.o
 libperf-y += term.o
+libperf-y += help-unknown-cmd.o
 
 libperf-$(CONFIG_LIBBPF) += bpf-loader.o
 libperf-$(CONFIG_BPF_PROLOGUE) += bpf-prologue.o