tftpd: introduce new option -S

Historically, tftpd disallowed write requests to existing files
that are not publicly writable. Such requirement is questionable at least.
Let us make it possible to run tftpd in chrooted environment
keeping files non-world writable.

New option -S enables write requests to existing files
for chrooted run according to generic file permissions.
It is ignored unless tftpd runs chrooted.

Requested by: marck
Differential: https://reviews.freebsd.org/D41090 (based on)

(cherry picked from commit 273a307d0b80743fb08e23237b3f74dc94a8fa2a)
(cherry picked from commit 03c2616dc530e5b23f06f9aa421012154590e578)