returns # of bytes in c->Out remaining after all write operations; anything is written if (1) C_WANTWR is set AND (2) c->Out.total_bytes > 0 after encryption AND (3) C_NOWR is not set if c->Out.total_bytes becomes 0, C_WANTWR is cleared ("nothing to write") and C_WANTRD is set if c->Out.total_bytes remains >0, C_WANTRD is cleared ("stop reading
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