MOSH_ESCAPE_KEY requires literal control characters

Currently, the implementation of MOSH_ESCAPE_KEY simply uses the first character of the environment variable as the escape key. This means that control characters must be encoded as literals in this variable. This makes it somewhat harder to set the variable in the first place, is confusing to users when non-printing control characters make the variable look empty when they do "echo $MOSH_ESCAPE_KEY", and poses a mostly-unlikely issue that users could cause themselves more pain if they set the variable to a control character that's interpreted by their terminal on output (ESC, TAB, CR, LF, ...)
I think it might be worthwhile to improve this before release, by having a little bit of syntax, maybe just "^B". That has a few issues of its own, though. This is tied in with the i18n issue of ASCII control characters often being hard to use or understand for anybody whose language varies much from English. @rinne, do you have any comments on this?

Start by locating the MOSH_ESCAPE_KEY implementation and reviewing how it currently takes the first environment-variable character. Evaluate the proposed control-character syntax such as "^B", including the stated terminal-output and internationalization concerns. The issue does not define a settled syntax or acceptance criteria, so completion would require agreement on the design.