libc: make setrlimit/getrlimit consistent and stop aborting

[gburd]

What

Fixes two problems with the resource-limit functions:

1. getrlimit() called abort() on any resource it did not special-case
   (RLIMIT_LOCKS, RLIMIT_SIGPENDING, RLIMIT_RTPRIO, RLIMIT_RTTIME). A
   program querying one of those crashed the whole unikernel, the same class of
   hazard as the old libaio stubs.

2. setrlimit() was a no-op that returned success, so a later getrlimit()
   reported the old (usually infinite) value rather than what the application
   just set, an inconsistency some programs depend on.

How

Replace the getrlimit() switch with a process-wide stored table of
RLIMIT_NLIMITS entries, initialized once with the previous defaults (stack
size from the default pthread attr, RLIMIT_NOFILE 10240, RLIMIT_NICE 0,
everything else infinite). getrlimit() reads the table, setrlimit() writes
it, both guarded by a mutex. Out-of-range resources and rlim_cur > rlim_max
return EINVAL instead of crashing or being silently accepted.

The limits remain advisory (OSv is a single address space and does not enforce
most of them), but set and get are now consistent.

Wire setrlimit as syscall SYS_setrlimit (only getrlimit and prlimit64
were in the table) with a tracepoint.

Testing

tests/tst-rlimit.cc covers querying every resource without aborting, the
EINVAL paths, set/get round-trips, and prlimit. Passes on OSv under KVM.

(Recreated from #1415, which GitHub auto-closed when its branch was rebased onto current master. Same change, rebased and verified on master 3aba46c.)