support of -p file command-line parameter

Here's a patch that adds back the `-p` option, using imlib2 for image
reading.  Thus, it supports everything imlib2 knows about, such as tga, gif,
png, jpeg, tiff, gif and others.

Warning: I don't know anything about X11 development. The patch works, but
it might not follow best practices.
various style and compiler warning changes
add Imlib, remove read.c, for "-p" option (background image)