gdb: error if 'thread' or 'task' keywords are overused

gdb: error if 'thread' or 'task' keywords are overused

When creating a breakpoint or watchpoint, the 'thread' and 'task'
keywords can be used to create a thread or task specific breakpoint or
watchpoint.

Currently, a thread or task specific breakpoint can only apply for a
single thread or task, if multiple threads or tasks are specified when
creating the breakpoint (or watchpoint), then the last specified id
will be used.

The exception to the above is that when the 'thread' keyword is used
during the creation of a watchpoint, GDB will give an error if
'thread' is given more than once.

In this commit I propose making this behaviour consistent, if the
'thread' or 'task' keywords are used more than once when creating
either a breakpoint or watchpoint, then GDB will give an error.

I haven't updated the manual, we don't explicitly say that these
keywords can be repeated, and (to me), given the keyword takes a
single id, I don't think it makes much sense to repeat the keyword.
As such, I see this more as adding a missing error to GDB, rather than
making some big change.  However, I have added an entry to the NEWS
file as I guess it is possible that some people might hit this new
error with an existing (I claim, badly written) GDB script.

I've added some new tests to check for the new error.

Just one test needed updating, gdb.linespec/keywords.exp, this test
did use the 'thread' keyword twice, and expected the breakpoint to be
created.  Looking at what this test was for though, it was checking
the use of '-force-condition', and I don't think that being able to
repeat 'thread' was actually a critical part of this test.

As such, I've updated this test to expect the error when 'thread' is
repeated.