some manpage reference cleanups

Hello,

there are some references to manpages that should be reviewed.

slstat(8) refers to ststat(1) but means systat(1). This typo crept in 
nine years ago with version 1.4 of src/usr.sbin/slstat/slstat.8.

da(4) refers two times to bsdlabel(5) which has been exempted some years 
ago.

There is a reference to a manpage p1003_1b(9) in hier(7) that has never 
existed for the last six years.

vinvalbuf(9) refers to brelse(9) and bremfree(9) but there are no such 
manpages for the last four years. Maybe somebody will write them in 
future. :)

periodic.conf(5) refers to rdist(1) which was removed four years ago. In 
fact the whole paragraph concerning "daily_distfile_enable" is obsolete.

intro(6) refers to dm(8) which was removed three years ago.

ports(7) refers to pib(1) which probably means the program sysutils/pib 
from ports collection, but this port doesn't install a manpage.

fixmount(8) refers to rmtab(5) but this manpage exist nowhere. Maybe you 
want to leave this untouched, because it's contributed.

libarchive-formats(3) should refer to mkisofs(8) instead of mkisofs(1)

vinum(8) is still referred by ccdconfig(8), ffsinfo(8), gconcat(8), 
gmirror(8), graid3(8), gstripe(8), growfs(8), newfs(8), ccd(4), 
vinum(4), tuning(7). I suppose we need a gvinum(8).

There are some further references that you may want to remove or fix:

- security(7) -> ksu(1)
- kiconv(3) -> iconv(9)
- mount_smbfs(8) -> nsmb(8)
- device_set_driver(9) -> isa(4)
- pam_opie(8) -> opiechallenge(3)
- pf.conf(5) -> securelevel(7)


Regards Björn