`get_dumpable` shouldn't return bool

[r-chiba]

Hello,

nix::sys::prctl::get_dumpable() returns Result<bool> but prctl(PR_GET_DUMPABLE) 1 could return SUID_DUMP_DISABLE (0), SUID_DUMP_USER (1) or SUID_DUMP_ROOT (2) 2.

Can we fix this function to return Result<i32> (or a new enum for SUID_DUMP_*)?

Please note that nix::sys::prctl::set_dumpable() is fine to receive a bool value because the argument is checked to be SUID_DUMP_DISABLE or SUID_DUMP_USER 3.