Add "%" prefix for syscall classes in qualify

Add "%" prefix for syscall classes in qualify

(as suggested in
https://www.mail-archive.com/strace-devel@lists.sourceforge.net/msg05147.html )

In order to move them to a different namespace than syscall names. This is
already a problem in case of ipc class which shadows ipc syscall.

Old behaviour is retained in order to preserve backwards compatibility.

* qualify.c (lookup_class) <syscall_class>: Add %-prefixed class definitions.
* strace.1 (.SS Filtering): Add information regarding %-prefixed class
syntax, declare legacy class syntax deprecated.
* tests/net.test: Update to use %-prefixed syscall class name.
* tests/netlink_protocol.test: Likewise.
* tests/qual_fault-exit_group.test: Likewise.
* tests/qual_syscall.test: Likewise.
* tests/scm_rights-fd.test: Likewise.
* tests/sigreturn.test: Likewise.
* tests/uio.test: Likewise.
* NEWS: Mention this change.