New option to allow use of the local OS username for login to the remote side

New option to allow use of the local OS username for login to the remote side
if we have no better ideas, with UI shamelessly stolen from Quest PuTTY.

Off by default, which effectively reverts the change to using the local
username by default that came in with GSSAPI support in r8138. Anyone wanting
seamless single sign-on will need to set the new option. (The previous
default behaviour was getting in the way in ad-hoc scenarios.)

Note that the PSCP and Unix-Plink behaviour of using the local username by
default have remained unchanged throughout; they are not affected by the new
option. Not sure if that's the Right Thing.

[originally from svn r8324]
[r8138 == de5dd9d65c0e2ffa9bc9cab038bf0be77efbf820]