Detect and reject duplicate internal code prefixes

Don't let (or at least try to prevent) a sysop create a duplicate message group or file library internal code prefix (which could lead to duplicate internal codes for subs and dirs, pretty easily and accidentally).
14 jobs for master in 9 minutes and 36 seconds (queued for 13 seconds)
Status Job ID Name Coverage
  Build
passed #30054
FreeBSD
jsdoor-freebsd

00:04:09

passed #30055
Linux
jsdoor-linux

00:06:38

passed #30057
FreeBSD
jsdoor-windows

00:01:58

passed #30049
FreeBSD
sbbs-freebsd

00:05:42

passed #30044
Linux
sbbs-linux

00:09:30

passed #30047
Windows
sbbs-windows

00:05:56

passed #30056
FreeBSD
sexpots-freebsd

00:00:13

passed #30045
Linux
sexpots-linux

00:00:23

passed #30048
Windows
sexpots-windows

00:00:40

passed #30052
FreeBSD
syncdraw-freebsd

00:00:30

passed #30053
Linux
syncdraw-linux

00:01:06

passed #30050
FreeBSD
syncterm-freebsd

00:01:07

passed #30046
Linux
syncterm-linux

00:02:45

passed #30051
FreeBSD
syncterm-windows

00:01:51