abcl: fix loading asdf systems from jar files

Under ABCL, when loading system definitions from a JAR-PATHNAME, the
PARSE-UNIX-NAMESTRING was including the DEVICE in the defaults to
MAKE-PATHNAME. This was mostly harmless but fails when (the as yet
unlrelease abcl-1.8.1) tightened the definition of JAR-PATHNAME to
always have an absolute directory
Status Name Job ID Coverage
  Test
passed Standard pipeline