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.

MFC after: 1 month
Requested by: marck
Differential: https://reviews.freebsd.org/D41090 (based on)