usrmerge breaks POSIX

[Vincent Lefevre <vincent@vinc17.net>]

POSIX says:

  SHELL   This variable shall represent a pathname of the user's
          preferred command language interpreter. If this interpreter
          does not conform to the Shell Command Language in XCU
          Chapter 2 (on page 2345), utilities may behave differently
          from those described in POSIX.1-2017.

There is no requirement to match one of the /etc/shells pathnames.
The user or scripts should be free to use any arbitrary pathname to
the command language interpreter available on the system, and Debian
should ensure that this is allowed, in particular the one give by
the realpath command.

-- 
Vincent Lefèvre <vincent@vinc17.net> - Web: <https://www.vinc17.net/>
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Work: CR INRIA - computer arithmetic / AriC project (LIP, ENS-Lyon)