add 'file cat' to output a stored file (with our without variable replacement) to stdout

+ add file_cat function
+ network_ip_unassign also unreserves and otherwise unallocates any IP
+ add verbose option to parse_template to output missing or undefined variables
+ system functions use file_cat instead of duplicating functionality
* bugfix handling of constant values with an '&' or '/' character
* switch constant storage to lowercase for consistency with actual usage
* bugfix join handling when deduplicating constant values
* bugfix host ip assignment when an ip is reserved
* bugfix system_vars to avoid reassigning C variable, since it is commonly used elsewhere
* rename kvm-uuid.sh to kvm-uuid