gdb_server: allow multiple GDB connections to selected targets

gdb_server: allow multiple GDB connections to selected targets

The default way of working is to have a single GDB attached to one
target, so OpenOCD accepts only one connection to the GDB port of
each targets and rejects any further connection.

There are some barely safe use cases in which it could get useful
having a second GDB connection to the same target.
One such use case is while using GDB as a 'non-intrusive memory
inspector', as explained in the OpenOCD documentation.
One GDB can be left running an infinite loop to dump some memory
area, or even analysing the content, while keeping a second GDB
ready for user interaction or spot memory check.

Add a target configure option to specify the maximum number of GDB
connections allowed for that target, keeping the default to 1.

Change-Id: I4985a602e61588df0b527d2f2aa5b955c93e125e
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5865
Tested-by: jenkins
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>