networkd: fix and simplify format_lifetime()

We would copy "forever" into the buffer. This is a fairly common case, so let's
do a microoptimization and return a static string. (All callers use the return
pointer, so this works just as well.)

The prefix "for " was not displayed, because the pointer to the part of the
buffer after "for " was returned. (Maybe it's just me, but I find strpcpy()
and associated functions really hard to use… I always have to look up what the
do exactly and what the return value is.)

A simple test is added.