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>