In mysql.php ignore SIGINT

In mysql.php ignore SIGINT

Every foreground process with the terminal open will receive SIGINT when
the interrupt key is pressed. But mysql handles SIGINT and kills the
running query, we don't want the wrapper to exit in that case. So,
ignore SIGINT while mysql is running. This is similar to how a shell
deals with this problem.

The other options would have been:

* Close the filehandles in PHP after forking (probably impossible)
* Use pcntl_exec() so that PHP isn't running (more complicated and
  requires the pcntl extension anyway)
* Clear termcap ISIG flag (probably would have broken mysql)

Bug: T199152
Change-Id: I7660cd24e036fbe3fe36d12563ffe1c1fa2360d0