testsuite: Introduce dejagnu_version

testsuite: Introduce dejagnu_version

The next patch will require checking the DejaGnu version.  There is
already a test that does this,
gdb.threads/attach-many-short-lived-threads.exp.  This patch introduces
a new procedure, dejagnu_version, and makes that test use it.

The version number is "right-padded" with zeroes, to make sure that we
always return a triplet (major, minor, patch).

The procedure does not consider the DejaGnu versions from git.  For
example, if you used DejaGnu from its current master branch, the version
would be "1.6.1-git", meaning that 1.6.1 will be the next release.  I
figured we'll cross that bridge when (and if) we get there.

gdb/testsuite/ChangeLog:

	* lib/gdb.exp (dejagnu_version): New proc.
	* gdb.threads/attach-many-short-lived-threads.exp (bad_dejagnu):
	Use dejagnu_version.