htpasswd: Use correct file mode for checking if file is writable.

Also switch to the non-deprecated APR_FOPEN_* flags.
PR: 45923

Backport of r1369618 from trunk.

Submitted by: sf
Reviewed by: rjung, trawick, humbedooh
Backported by: rjung


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1372373 13f79535-47bb-0310-9956-ffa450edef68