Bug 4969: GCC-9 build failure: stringop-truncation (#441)

Bug 4969: GCC-9 build failure: stringop-truncation (#441)

GCC-9 with Squid use of -Werror makes these warning hard
errors which can no longer be ignored. We are thus required
to alter this third-party code when built for Squid.

Truncation of these strings is fine. Rather than suppress
GCC warnings, switch to xstrncpy() which has similar
behaviour but guarantees c-string terminator exists within
the copied range limit (removing need for two -1 hacks).

This change will add terminators on path and device_type
values in the rare case of overly long configured values.
It is not clear what ancient Domain Controllers would do
when handed un-terminated c-string in those cases, but was
unlikely to be good.