install: Fix COMPONENT option

Since commit 7ced0732 (make default install component name configurable,
2012-05-13) the COMPONENT option has not worked in all cases.  The
change restored use of the broken SetDefaultString previously removed by
commit 43cad3e4 (fix the default "Unspecified" component, 2008-03-26).

Remove SetDefaultString and use the default component name only when an
invocation does not specify a non-empty COMPONENT option value.

Bug-Debian: http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=680825