close #4463

For the line "Host host1 # this is a comment", the current
implementation generates words in an inline comment as hostnames.
This patch removes the comment before generating the hostname.

In ~/.ssh/config, "=" can also be used as a separator between the
field name and the value.  The current master does not properly handle
this and generate a hostname "=" or one starting with "=".  This patch
correctly handles it.

Comments are anyway removed in the subsequent call to `sub(/#.*/,
"")`, and it becomes a blank line.  Blank lines do not have fields, so
they are ignored in the next for-loop.

In the current implementation, any hostnames in /etc/hosts containing
"0.0.0.0" as a part (such as "110.0.0.0" would be excluded.  "0.0.0.0"
should be checked by the exact match.

An entry of the form `[example.com]:port,192.168.0.1 ...` in
~/.ssh/known_hosts are not properly processed.  The current
implementation gives up the matching on the first occurrence of `]`,
the subsequent 192.168.0.1 would not be extracted.  This patch
continues the analysis and removes "]" together with "[".

This patch also removes the ":port" part from the hostnames in
~/.ssh/known_hosts.  One cannot use the form "hostname:port" in the
arguments to the ssh command anyway.