fix call to err()
handle select errors EAGAIN | EWOULDBLOCK in lpm
handle some should-be impossible paths
guard swapcontext in _native_syscall_leave
handle more errors and remove race condition in native_isr_entry
Don't unblock signals again.

A late signal could kill the process otherwise.
guard setitimer
block all signals per default
force-debug (eINT && in_isr) in DEVELHELP
dont err() but warn() if no thread exists in native_isr_entry