* xstring.cc, xstring.h: shrink buffer explicitly.
* PollVec.cc, PollVec.h: use select; (FDReady) new method.
* TimeDate.cc, TimeDate.h: change to microsecond resolution.
* Timer.cc, Timer.h: (GetTimeoutTV) new method.
* SMTask.cc, SMTask.h: use Timer::GetTimeoutTV; add resumed task to the tail of new_tasks list.
* buffer.cc, buffer.h: change buffer size tuning; remove last_got member; don't call down->Do in BufferStacked; use last select result to save a read() system call.
* ftpclass.h: don't suspend control_recv so that control_sock gets polled when delaying data read ops.
* CharReader.cc: check last select result before reading.
* FileCopy.cc: use microseconds for delaying reads; don't reset delay on data unavailability; don't increase delay when read returned enough data.
log changes