dovecot: Fix `jail -m jid=NAME` on 13.2

[tuffnatty]

On FreeBSD 13.2, passing a non-numeric value for jid results in error:

# jail -m jid=stage allow.sysvipc=1
jail: jid: non-integer value "stage"

Changes proposed in this pull request:

• Use jail -m name=NAME

Checklist:

• [ ] docs up-to-date