updated pnd_make.sh based on comments from the ML and irc:

updated pnd_make.sh based on comments from the ML and irc:
* use genpxml.sh and the schema from the dir in which pnd_make.sh is if
* no param is supplied
* check if a PXML file was actually created during the genpxml step
* add some checks for required programs (iso and squashfs stuff)
* add some more sanity checks including aborts if an error was detected
* add missing help info for the param --genpxml
* add comment at the top of the file mentioning what the file is about
* fix various typos