Use getent instead of grepping /etc/passwd.

Use getent instead of grepping /etc/passwd.

Forthcoming versions of cygwin will use a different mechanism for
obtaining passwd/group information based on /etc/nsswitch.conf.
Thus, it will not be guaranteed that the files /etc/passwd and
/etc/group even exist. The recommended way for obtaining those
info is by using the getent command, which already works in
current versions.