Report _SC_SEM_NSEMS_MAX and _SC_SEM_VALUE_MAX which show parameters
of the current usermode implementation of the POSIX semaphores.

For NSEMS_MAX, return -1 without changing errno, which indicates that
the variable has no limit. Before, sysconf(3) returned parameters
queried from the ksem(9) legacy implementation, which apparently has
low defaults for NSEMS_MAX.

Reported and tested by: jbeich
Sponsored by: The FreeBSD Foundation
MFC after: 1 week