libnetmap: remove interface name validation

When trying to use a VLAN device (e.g. "em0.123") with a dot
the library fails to parse the interface correctly. The former
pattern is much too restrictive given that almost all characters
can be coerced into a device name via ifconfig.

Remove the particularly restrictive validation. Some characters
still cannot be used as an interface name as they are used as
delimiters in the syntax, but this allows to be able to use most
of them without an issue.

Submitted by: franco@opnsense.org
Differential Revision: https://reviews.freebsd.org/D42485
Reviewed by: vmaffione