Introduce and use new astrcat() and astrcatf() functions which replace
the strbuf functionality for subcmd.

For now they duplicate strbuf's die-on-allocation-error policy.
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/957d207e1254406fa11fc2e405e75a7e405aad8f.1450193761.git.jpoimboe@redhat.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>