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.

Submitted by: yocalebo_gmail.com
Reviewed by: rmacklem
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D32993