Improve CLI --entryslot

[pstef]

Description

For the change introduced in 4cdcaa2 to make sense, the called strto function should be the variant that returns a signed type and that returned value should not be cast to unsigned.

Reviewers

@sonninnos

[sonninnos]

It was never the intention to allow loading the auto state slot with -e -1, as the very next row shows.

[pstef]

$ ./retroarch -e abc 2>&1 | fgrep entryslot
[WARN] [State]: --entryslot argument "abc" is not a valid entry state slot index. Ignoring.
$ ./retroarch -e -1 2>&1 | fgrep entryslot
[WARN] [State]: --entryslot argument "-1" is not a valid entry state slot index. Ignoring.
$ ./retroarch -e 0 2>&1 | fgrep entryslot
$ ./retroarch -e 1 2>&1 | fgrep entryslot
$ ./retroarch -e 999 2>&1 | fgrep entryslot
$ ./retroarch -e 1000 2>&1 | fgrep entryslot
[WARN] [State]: --entryslot argument "1000" is not a valid entry state slot index. Ignoring.
$ ./retroarch -e 1a 2>&1 | fgrep entryslot
[WARN] [State]: --entryslot argument "1a" is not a valid entry state slot index. Ignoring.