Add an optional "alias" attribute to syscall entries.

Add an optional "alias" attribute to syscall entries.

When setting a syscall catchpoint by name, catch syscalls whose name
or alias matches the requested string.

When the ABI of a system call is changed in the FreeBSD kernel, this
is implemented by leaving a compatibility system call using the old
ABI at the existing "slot" and allocating a new system call for the
version using the new ABI.  For example, new fields were added to the
'struct kevent' used by the kevent() system call in FreeBSD 12.  The
previous kevent() system call in FreeBSD 12 kernels is now called
freebsd11_kevent() and is still used by older binaries compiled
against the older ABI.  The freebsd11_kevent() system call can be
tagged with an "alias" attribute of "kevent" permitting 'catch syscall
kevent' to catch both system calls and providing the expected user
behavior for both old and new binaries.  It also provides the expected
behavior if GDB is compiled on an older host (such as a FreeBSD 11
host).

gdb/ChangeLog:

	* NEWS: Add entry documenting system call aliases.
	* break-catch-syscall.c (catch_syscall_split_args): Pass 'result'
	to get_syscalls_by_name.
	* gdbarch.sh (UNKNOWN_SYSCALL): Remove.
	* gdbarch.h: Regenerate.
	* syscalls/gdb-syscalls.dtd (syscall): Add alias attribute.
	* xml-syscall.c [!HAVE_LIBEXPAT] (get_syscalls_by_name): Rename
	from get_syscall_by_name.  Now accepts a pointer to a vector of
	integers and returns a bool.
	[HAVE_LIBEXPAT] (struct syscall_desc): Add alias member.
	(syscall_create_syscall_desc): Add alias parameter and pass it to
	syscall_desc constructor.
	(syscall_start_syscall): Handle alias attribute.
	(syscall_attr): Add alias attribute.
	(xml_get_syscalls_by_name): Rename from xml_get_syscall_number.
	Now accepts a pointer to a vector of integers and returns a
	bool.  Add syscalls whose alias or name matches the requested
	name.
	(get_syscalls_by_name): Rename from get_syscall_by_name.  Now
	accepts a pointer to a vector of integers and returns a bool.
	* xml-syscall.h (get_syscalls_by_name): Likewise.

gdb/doc/ChangeLog:

	* gdb.texinfo (Set Catchpoints): Add an anchor for 'catch syscall'.
	(Native): Add a FreeBSD subsection.
	(FreeBSD): Document use of system call aliases for compatibility
	system calls.