conf: fix clang warning when building w/o libcap

conf: fix clang warning when building w/o libcap

when compiling lxc with clang-5.0 parse_cap()'s main loop will produce a
warning about a tautological comparision (#2215).

By moving the result of computation into a variable (end) this is no
longer a constant expression. clang-5.0 does not do dataflow analysis at
this point, so it is, to quote someone from #llvm, "morally equivalent"
to casting `(int)i`.

in addition, we also clean up the #if HAVE_LIBCAP to no longer need
its #else branch!

Signed-off-by: Igor Galić <igor.galic@automatic-server.com>