target: make "examined" flag be per-target

target: make "examined" flag be per-target

Previously this flag was stored in "target_type", so that for example
if there were two ARM7TDMI targets in a scan chain, both would claim
to have been examined although only the first one actually had its
examine() method called.

Move this state to where it should have been in the first place, and
hide a method that didn't need exposure ... the flag is write-once.

Provide some doxygen.  The examine() method is confusing, since it
isn't separating one-time setup from the after-each-reset stuff.  And
the ARM7/ARM9 version is, somewhat undesirably, not leaving the debug
state alone after reset ... probably more of an issue for trace setup
than for watchpoints and breakpoints.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>