rc: dmesg: Allow umask to be configurable

Allow umask to be configurable.

Being able to set the umask via an rc variable is useful when setting:

security.bsd.unprivileged_read_msgbuf=0

As it allows a user to configure:

dmesg_umask="066"

Without modifying the rc script, and preventing the contents of the
$dmesg_file (/var/run/dmesg.boot) from being publicly readable.

PR: 272552
Reviewed by: netchild
MFC after: 2 days
Differential Revision: https://reviews.freebsd.org/D53169