Factor out a JSch-independent ssh config parser

Factor out a JSch-independent ssh config parser

Move the bulk of the basic parsing and host entry handling into a
new class OpenSshConfigFile that has no dependencies on any concrete
ssh implementation. Make the existing OpenSshConfig use the new
parser.

Introduce a new class SshConstants collecting all the various ssh-
related string literals. Also use TreeMaps with a case-insensitive
key comparator instead of converting keys to uppercase. Add a test
to verify that keys are matched case-insensitively.

Most of the parsing code was simply moved, except that the new
parser supports looking up entries given host name, port, and user
name, and can thus handle more %-substitutions correctly. This
feature is not yet used and cannot be used with JSch since JSch
only has a ConfigRepository.getConfig(String) interface.

The split is still worth the trouble as it opens the way to using
another ssh client altogether. Apache MINA sshd, for instance,
resolves host entries giving host name, port, and user name.

(Apache MINA has a built-in ssh config handling, but that has
problems, too: its pattern matching is case-insensitive, and its
merging of host entries if several match is not the same as in
OpenSsh. But with this refactoring, it will be possible to plug in
OpenSshConfigFile into an Apache MINA sshd  client without dragging
along JSch.)

One test case that doesn't make sense anymore has been removed. It
tested that repeatedly querying for a host entry returned the same
object. That is no longer true since the caching has been moved to
a deeper level.

Bug: 520927
Change-Id: I6381d52b29099595e6eaf8b05c786aeeaefbf9cc
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>