After commit 2049e97d ("dhcp: refactor parsing of 'request' and
'also request' options") NM parses all the existing 'request' and
'also request' from the original configuration file and appends them
as 'also request' to avoid duplicates and conflicts.

So if the original file contains 'request x' (which means "request
only option x instead of builtin defaults"), we would translate it
into 'also request x', which appends the option to the builtin
defaults, causing duplicates in the DHCP request as dhclient seems not
smart enough to sanitize the list by itself.

To fix this, ensure that the request list is reset if the
configuration file contains a 'request'.

Fixes: 2049e97d

https://bugzilla.gnome.org/show_bug.cgi?id=778430