Spelling fixes.

Corrected typo in documentation for R_Zero, R_PosInf, R_NegInf, R_Nan;

Fixed compilation on Mac/PPC

[issue 620] don't segfault if number of arguments is less than 1

Emit a log message when AOF fsync is still in progress but we are forced to write from the main thread since two seconds already elapsed.

fixed flushAppendOnlyFile() function prototype to reflect the new implementation, fixing compilation.

Added aof_backgronud_fsync() function, and use it in the bacground rewrite done handler when the fsync policy is everysec.

fixed a problem in bioOlderJobType() when there are no jobs of the specified type. Now the function returns 0 when this happens.

Background I/O library enhanced so that the main thread can query for the number of pending jobs of the specified type.

Added note about Tcl

Optimize LRANGE to scan the list starting from the head or the tail in order to traverse the minimal number of elements. Thanks to Didier Spezia for noticing the problem and providing a patch.

use bio.c instead of libeio for closing the AOF file in background. Some comment added and other minor changes.