net/samba419: Dynamically calculate FreeBSD proc_fd_pattern

Port timur's patch for dynamic calcuation of the proc fd (fdescfs)
pattern mounted by the samba_server RC script from net/samba416.

Tested by: michaelo
Approved by: jrm (mentor), mikael, timur
MFH: 2025Q1
Differential Revision: https://reviews.freebsd.org/D48416

(cherry picked from commit f3ab3f3b2b7166cc33007f454b744123382c4b8a)