Implement io_pgetevents syscall decoding

Implement io_pgetevents syscall decoding

Introduced by Linux commit v4.17-rc3-7-g7a074e9.

* print_aio_sigset.c: New file.
* Makefile.am (strace_SOURCES): Add it.
* aio.c (print_io_getevents): Rename from SYS_FUNC(io_getevents), add
has_usig flag, print user sigset on exiting if has_usig is set to true.
(SYS_FUNC(io_getevents), SYS_FUNC(io_pgetevents)): New functions,
wrappers for print_io_getevents.
* linux/32/syscallent.h ([292]): Wire up io_pgetevents.
* linux/64/syscallent.h ([292]): Likewise.
* linux/i386/syscallent.h ([385]): Likewise.
* linux/x32/syscallent.h ([333]): Likewise.
* linux/x86_64/syscallent.h ([333]): Likewise.
* NEWS: Mention this improvement.

Co-Authored-by: Dmitry V. Levin <ldv@altlinux.org>