Build fails: strncpy’ output truncated before terminating nul copying as many bytes from a string as its length

[Rot127]

Build fails for newer gcc versions (I think beginning from ver. 8) with:
strncpy’ output truncated before terminating nul copying as many bytes from a string as its length [-Werror=stringop-truncation]
for the files:

• gre2tun.c
• tun2gre.c
• tundev.c

Workaround: Replace strncpy with memcpy.

Though this doesn't solve the truncation problem here!