util: Ensure debugger can be attached to process

util: Ensure debugger can be attached to process

samba_start_debugger() attempts to start a debugger attached to the
calling process by calling system() to start a background process.
However, if the spawned shell exits before the debugger has had a chance
to attach, the debugger process will no longer be a child of the parent
process (as it will have been reparented).

If the system does not allow tracing by non-child processes, attachment
may fail as a result.

This commit replaces the system() call and the implicit shell around
xterm with an explicit fork()/exec() so that the debugger remains a
child of the calling process, ensuring the attachment succeeds unless
tracing is disabled completely.

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Andrew Bartlet <abartlet@samba.org>
Reviewed-by: Andreas Schneider <asn@samba.org>