configure: improve shell search-and-re-execute logic.

* configure: If we determine we are running on Bash, then do
not re-execute. If re-executing, then look for CONFIG_SHELL
first, which is an Autoconf convention that some distros rely
on. Include dash in the fallback shell list because dash works
fine. Include more places in which to look for bash and dash.
Uses of txr_shell must be quoted now because it could take on
the value of CONFIG_SHELL which we don't control.
Print a trace message about re-executing. Print the message
about which shell we are running on earlier, before
parsing the variables and printing the baner.
Since we might not re-execute any more, we might not know the
exact name of the shell we are running on. That is difficult
and hacky to obtain, so instead we print an alternative
message that we are running on the original shell.
(gen_config_make): Only generate the SHELL Makefile variable
if txr_shell is set. If whatever shell we were run with is
good enough for the configure script, it's good enough for
Makefile recipes. Likely, the configure script was executed
directly, so that /bin/sh is that default shell and that is
what GNU Make will use by default.

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