DrSyscall: Improve Linux argument naming

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[This DrMemory Users' Group Thread](https://groups.google.com/g/drmemory-users/c/xUsYnvgnivg)

**Is your feature request related to a problem? Please describe.**
I've been building a Linux-main tracer client, whose one of the task is logging syscalls using `drsys_iterate_args()` and noticed that `drsys_arg_t.arg_name` is always set to NULL for Linux syscalls. That's because `syscall_info_t` doesn't expose any parameter names.

**Describe the solution you'd like**
The most simple and self-documenting tweak would be to add an `arg_name` field to `sysinfo_arg_t`. This way, instead of writing `"handle_xyz"` for every syscall, arg names essentially become part of the docs:
```c
// before
{{PACKNUM(86,9,9,-1),0},"link", OK, RLONG, 2,
{
{0,0, R|CT, CSTRING},
{1,0, R|CT, CSTRING},
}
}
// after
{{PACKNUM(86,9,9,-1),0},"link", OK, RLONG, 2,
{
{0,0, R|CT, CSTRING, .arg_name = "oldname"},
{1,0, R|CT, CSTRING, .arg_name = "newname"},
}
}
```
It can be also expanded by adding names for the previously omitted inlined scalars:
```c
// before
{{PACKNUM(9,-1,-1,SYS_mmap),0}, "mmap", OK, RLONG, 6,}
// after
{{PACKNUM(9,-1,-1,SYS_mmap),0}, "mmap", OK, RLONG, 6,
{
{0, sizeof(void*), SYSARG_INLINED, DRSYS_TYPE_POINTER, .arg_name = "addr"},
{1, sizeof(size_t), SYSARG_INLINED, DRSYS_TYPE_SIZE_T, .arg_name = "length"},
{2, sizeof(int), SYSARG_INLINED, DRSYS_TYPE_SIGNED_INT, .arg_name = "prot"},
{3, sizeof(int), SYSARG_INLINED, DRSYS_TYPE_SIGNED_INT, .arg_name = "flags"},
{4, sizeof(int), SYSARG_INLINED, DRSYS_TYPE_SIGNED_INT, .arg_name = "fd"},
{5, sizeof(off_t), SYSARG_INLINED, DRSYS_TYPE_SIGNED_INT, .arg_name = "offset"},
}
```
In fact, this scheme (just without names) is already present in the code - other OSes denote full argument ranges, they're also present in the IOCTL sub-table and sometimes in the main Linux table (see e.g. `getrandom`, `rseq` and `syncfs`).

Do you have any implementation in mind for this feature?
Yes, currently working on it at my fork.

**Describe alternatives you've considered**
Since this is an API update proposal, I'll leave this field empty.

Start with drsys_iterate_args(), syscall_info_t, and sysinfo_arg_t, then inspect the Linux syscall tables, including getrandom, rseq, syncfs, and the IOCTL sub-table. Done means syscall arguments expose names through arg_name, including the previously omitted inline scalar ranges, while preserving the proposed API behavior.

Written by the indexing model from the issue text.