Change get_syscalls_by_group to append to an existing vector of integers.

Change get_syscalls_by_group to append to an existing vector of integers.

This removes the need for the caller to explicitly manage the memory
for the returned system call list.  The sole caller only needed the
system call numbers rather than the full syscall structures.

get_syscalls_by_group now uses a boolean return value to indicate if
the requested group exists.

gdb/ChangeLog:

	* break-catch-syscall.c (catch_syscall_split_args): Pass 'result'
	to get_syscalls_by_group.
	* xml-syscall.c [!HAVE_LIBEXPAT] (get_syscalls_by_group): Return
	false.
	[HAVE_LIBEXPAT] (xml_list_syscalls_by_group): Append syscall
	numbers to an existing vector of integers and return a bool.
	(get_syscalls_by_group): Accept pointer to vector of integers
	and change return type to bool.
	* xml-syscall.h (get_syscalls_by_group): Likewise.