network: use a common helper to parse bounded ranges

network: use a common helper to parse bounded ranges

This compresses repetetive code and makes it easier to add new options
in networkd. The formatting of error messages becomes uniform. The
error message always specifies the rvalue literally, instead of using
a "descriptive name". This makes the message much easier to handle for
the user.

I opted to add just one parser, and wrap it with inline functions to proxy
the type. This is less verbose than copying functions for each type
separately, and the compiler should be able to get rid of the inline wrapper
almost entirely.

asserts are reordered to use the same order as the parameter list.
This makes the code easier to read.

No functional change intended, apart from the difference in error message
formatting.