common: Fix gsu_is_a_number().

We must use [ ... ] in gsu_is_a_number() because arithmetic expansion
with ((...))  undergoes parameter and variable expansion. For example

	x=foo
	gsu_is_a_number "$x"

will pass "foo" to gsu_is_a_number() where the expression  (("$1"))
tries to expand the (non-existing) variable foo, aborting the script
if set -u was given.

This bug was introduced a few month ago in commit 44860e92 (Use modern
style arithmetic evaluation everwhere).