Teach configure --editline to report itself to shell.c as either HAVE_EDITLINE or...

Teach configure --editline to report itself to shell.c as either HAVE_EDITLINE or HAVE_READLINE, depending on which headers are available (editline/readline.h or readline/readline.h, both of which work and some systems only have the latter), but then link against libedit regardless of which one shell.c thinks it is using.

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