perf tools: Provide subcmd configuration at runtime

Create init functions for exec_cmd.c and pager.c.  This allows their
configuration to be specified at runtime so they can be split out into a
separate library which can be used by other programs.  Their
configuration is stored in a shared subcmd_config struct.

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/21f5f6b38da72c985a8dcfa185700d03e7eecd1d.1450193761.git.jpoimboe@redhat.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>