Fix decoding of sgetmask and ssetmask syscalls

Fix decoding of sgetmask and ssetmask syscalls

Old ssetmask syscall takes an argument and returns a value of type int.
Old sgetmask syscall may return an error.

* signal.c (SYS_FUNC(ssetmask)): Explicitly convert the argument
and return value to unsigned int, print it using sprint_old_sigmask_val.
(SYS_FUNC(sgetmask)): Do not print the mask in case of syserror.
* tests/sxetmask.c: New file.
* tests/gen_tests.in (sxetmask): New entry.
* tests/pure_executables.list: Add sxetmask.
* tests/.gitignore: Likewise.
* NEWS: Mention this fix.