Util: add pidfile module.

This provides simple functions for creating, checking
and deleting pidfiles.
Util: add split_crab_vars function.

This parses CRAB variables from the start of a command string.
It is now used in importing crontabs and in crabsh.
Variables from the environment and those from the command
string are now merged by crabsh.

Also added a true_string function to check whether a string
should be taken to be a true value.  This avoids repeating the list
of false strings in various places.
Client: crabsh now responds to CRABIGNORE.

If this variable is set, it simply executes the command without
reporting to the Crab server.
Client: add PID file handling to crabsh.

It now accepts a --pidfile argument or CRABPIDFILE variable
to specifiy a PID file.  In this case it will use the file
to determine whether or not to start to job, sending the
ALREADYRUNNING finish status if it does not.