Fixed invalid time exception

UtcToUnixTimeStamp was throwing "System.ArgumentException: dateTime
parameter is an invalid time" for mktime(0,0,0,10,10,2010)
Implemented File Locking for Unix
Shell commands support for Unix
Fixed Working Directory for Unix

Mono + Apache|XSP don't set the working directory, thus new FullPath()
converts "bin" to "//bin" (mono 2.10.9) or "/bin".
More shell commands support for Unix
Fixed wrong Assert on Unix
Unix support

- Added PHP_OS (necessary for other commited patches)
- Fixed absolute-path-includes
GetFileMode support for Unix