pushing: add new get_user_uuid utility

pushing: add new get_user_uuid utility

As part of the http push user validation, the pushing user's
UUID is checked.  This utility retrieves the user's UUID but
rather than using getpwnam to retrieve it (which can only
check the system's or chroot's passwd file), it reads a
passwd file directly.

While the Girocco::User module can be used to retrieve the
same UUID as this utility, this utility avoids the need to
load the Perl interpreter just to retrieve the user's UUID.