pw: (file == NULL) check is always false in read_userconfig(), remove
it. Drop the now unused _PATH_PW_CONF definition. [1]

While here, change the last remaining hardcoded "/etc" to _PATH_PWD.

Noted by: glebius [1]
Reviewed by: eugen
Approved by: re (gjb), kib (mentor)
Differential Revision: https://reviews.freebsd.org/D17575