Converted link_poll_end from global to instance variable.
Fixes likely bug when multiple work_queue instances are present.
Converted short_timeout from global into an instance variable.
Moved selection of timeout into send_worker_msg for consistency and clarity.
Updated comment: catalog details have been fixed, code has been released.
Merge branch 'work-queue-global-cleanup' of git://github.com/dthain/cctools into dthain-work-queue-global-cleanup

Removed last two global variables in work_queue.c and converted into instance
variables.  Moved computation of message timeout into send_worker_msg for
consistency and clarity.
add option for controling the behaviour of explicit dependencies
long option for use explicit
move linker man page to makeflow_linker
add -e to man page
add -e to help display
make use_explicit a global option.

use_explicit was elevated to a global since it potentially affects every function.
allow explicit dependencies to be copied or skipped
add usage message
pass explicit dependency behavior to drivers
include option in test
add perl file type
list perl extensions
check for perl extension
first stab at perl driver
invoke perl driver for perl files
handle perl files in package structure and build package