Revert ServerAliveInterval change in 6.20161111, which caused problems
with too many old versions of ssh and unusual ssh configurations.
It should have not been needed anyway since ssh is supposted to
have TCPKeepAlive enabled by default.
So that stalled transfers will be noticed within about 3 minutes,
even if TCPKeepAlive is disabled or doesn't work.
Rather than setting with -o, use -F with another config file,
so that any settings in ~/.ssh/config or /etc/ssh/ssh_config overrides this.
key2file and file2key were top cost centers according to profiling.
The repeated use of replace was not efficient. This new approach is quite a
lot more efficient.
This commit was sponsored by Denis Dzyubenko on Patreon.
