Fix explicit casts of signed integer types to unsigned long long

Fix explicit casts of signed integer types to unsigned long long

* defs.h (widen_to_ull): New macro.
* dirent.c (print_old_dirent, SYS_FUNC(getdents)): Use it in place
of explicit casts to unsigned long long.
* io.c (print_lld_from_low_high_val): Likewise.
* lseek.c (SYS_FUNC(llseek)): Likewise.
* printsiginfo.c (print_si_info): Likewise.
* printstat.h (DO_PRINTSTAT): Likewise.
* sysinfo.c (SYS_FUNC(sysinfo)): Likewise.
* times.c (SYS_FUNC(times)): Likewise.
* fetch_struct_statfs.c (ASSIGN_NUMBER): Remove.
(fetch_struct_statfs, fetch_struct_statfs64): Replace ASSIGN_NUMBER
with widen_to_ull.