network: automatically determine timeout of waiting for carrier regain

network: automatically determine timeout of waiting for carrier regain

The commit 6706ce2fd2a13df0ae5e469b72d688eaf643dac4 made
IgnoreCarrierLoss= setting also take timespan, to make users handle
issues like #18738 or #20887. But still users needed to explicitly set
a timespan.

This makes networkd automatically determine the timeout when the
situations #18738 or #19832 is detected. Unfortunately, still users have
issue #20887 need to specify a value.

Closes #19832.