mountd: Fix handling of usernames that start with a digit

yocalebo_gmail.com submitted a patch for mountd.c that
fixes the case where a username starts with a digit.
Without this patch, the username that starts with a
digit is misinterpreted as a numeric uid.
With this patch, any string that does not entirely
convert to a decimal number via strtoul() is considered
a user/group name.

(cherry picked from commit f4bf849bb894f4934b8df6c04a820dfa52e9576c)