perf strlist: Allow substitutions from file contents in a given directory

perf strlist: Allow substitutions from file contents in a given directory

So, if we have an strlist equal to:

   "file,close"

And we call it as:

   struct strlist_config *config = { .dirname = "~/strace/groups", };
   struct strlist *slist = strlist__new("file, close", &config);

And we have:
  $ cat ~/strace/groups/file
  access
  open
  openat
  statfs

Then the resulting strlist will have these contents:

  [ "access", "open", "openat", "statfs", "close" ]

This will be used to implement strace syscall groups in 'perf trace',
but can be used in some other tool, thus being implemented in 'strlist'.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Borislav Petkov <bp@suse.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/n/tip-wi6l6qtomqlywwr6005jvs05@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>