Make pw_scan(3) more compatible with getpwent(3) et. al. when processing
data from /etc/passwd rather than /etc/master.passwd.

The libc getpwent(3) and related functions automatically read master.passwd
when run by root, or passwd when run by a non-root user. When run by non-
root, getpwent() copes with the missing data by setting the corresponding
fields in the passwd struct to known values (zeroes for numbers, or a
pointer to an empty string for literals). When libutil's pw_scan(3) was
used to parse a line without the root-accessible data, it was leaving
garbage in the corresponding fields.

These changes rename the static pw_init() function used by getpwent() and
friends to pw_initpwd(), and move it into pw_scan.c so that common init
code can be shared between libc and libutil. pw_scan(3) now calls
pw_initpwd() before __pw_scan(), just like the getpwent() family does, so
that reading an arbitrary passwd file in either format and parsing it with
pw_scan(3) returns the same results as getpwent(3) would.

This also adds a new pw_initpwd(3) function to libutil, so that code which
creates passwd structs from scratch in some manner that doesn't involve
pw_scan() can initialize the struct to the values expected by lots of
existing code, which doesn't expect to encounter NULL pointers or garbage
values in some fields.