Use BSD syntax (ps aux).

"ps -aux" is not a valid command but it may be (typically is)
interpreted as the famous BSD "ps aux" syntax.

"-aux" is invalid because -x is not a GNU flag and -u requires an
argument. If x where treated as the argument to -u the command's
semantics would be changed.
Do not send ps' column headers.

ps.php naively sends ps' complete output, including the column headers.
These aren't data and are already defined in the table anyway.

A more robust solution would probably be to map the headers to the
table since they conveniently identify the data. But this was easier
and we don't need to worry too much about robustness yet.
Merge pull request #54 from commonquail/ps-fix

Minor ps fixes