Changed option processing to use strtol() to get a umask

[tmwong2003]

get_mode()/s3fs_strtoofft() does not handle octal umask values, which
results in unexpected behavior when trying to set a world-readable umask
value.

[tmwong2003]

Incidentally, why does s3fs use a home-grown parser method strtoofft() instead of strtol() from cstdlib to convert decimal, octal, and hexadecimal strings to longs?

[ggtakec]

Thank you.
I was using conventionally, called a function that has been mainly used in part of other than this problem also on the parameters of the umask in s3fs.