utilities: yield if we fail to enter/acquire
posix: use UnlikelyLock instead of Mutex for the FD map lock, as we read FAR more often than we write, and UnlikelyLock is very, very fast for readers.
posix: cache local/remote paths for UNIX sockets in the descriptor object, as File::getFullPath is extraordinarily slow.
posix: #define UNIX_PATH_MAX in sys/un.h