Improve gcore shell quoting and portability

The gcore shell script (gdb/gcore.in) doesn't quote its variables
enough.

For example, trying to write a core file with - say - a space
ungraciously fails like this:

    $ gcore -o 'foo bar' 6270
    /usr/bin/gcore: line 92: [: foo: binary operator expected
    gcore: failed to create foo bar.6270

Similarly, one can inject meta characters like * (by accident)
that may yield unexpected results, e.g. as in:

    $ gcore -o foobar '*'

This change fixes these issues in several places.

Aso, since the script uses array syntax, the patch changes the
the shell in the first line from `/bin/sh` to /bin/bash`.

POSIX doesn't specify the array syntax for shell, thus, the
script doesn't work on systems where /bin/sh is linked to - say -
dash.

Since the source gcore.in already is processed by a pre-processor
one could even auto-detect the path to bash and thus dynamically
generate the first line. For systems where bash isn't available
via /bin/bash. But I think this would be overkill and /bin/bash
is good enough as most systems probably have it.

gdb/ChangeLog:

	PR gdb/22888
	* gcore.in: Quote variables and switch interpreter to bash.