Fix As_shell.try_exec. POSIXly redirect any output to /dev/null.

Not only where we redirecting only stderr, &> is not POSIX
compliant which breaks on linux Sys.command.
Add As_shell.has_cmd to test for the existence of a command.
Check ocamlfind existence using As_shell.has_cmd.
Merge pull request #100 from dbuenzli/fix-82

Beware of bashisms in Sys.command.