PR standards/42829
Implement parameter and arithmetic expansion of $ENV
before using it as the name of a file from which to
read startup commands for the shell. This continues
to happen for all interactive shells, and non-interactive
shells for which the posix option is not set (-o posix).
On any actual error, or if an attempt is made to use
command substitution, then the value of ENV is used
unchanged as the file name.
The expansion complies with POSIX XCU 2.5.3, though that
only requires parameter expansion - arithmetic expansion
is an extension (but for us, it is much easier to do, than
not to do, and it allows some weird stuff, if you're so
inclined....) Note that there is no ~ expansion (use $HOME).
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