fix #5410: config: fix naming scheme for names in firewall config

fix #5410: config: fix naming scheme for names in firewall config

This should bring the allowed names on par with the pve-firewall
naming scheme, allowing also underscores together with minus, as long
it isn't the first character [1].

[1] https://git.proxmox.com/?p=pve-firewall.git;a=blob;f=src/PVE/Firewall.pm;h=0abfeccffc94cec940760e69a894e392dc33f151;hb=29b48c381d14bf425232dc65c9c0d18f95c8f222#l51

Signed-off-by: Stefan Hanreich <s.hanreich@proxmox.com>
 [ TL: expand commit message slightly ]
Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>