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 Name Job ID Coverage
  Build
passed jsdoor-freebsd #30054
FreeBSD

00:04:09

passed jsdoor-linux #30055
Linux

00:06:38

passed jsdoor-windows #30057
FreeBSD

00:01:58

passed sbbs-freebsd #30049
FreeBSD

00:05:42

passed sbbs-linux #30044
Linux

00:09:30

passed sbbs-windows #30047
Windows

00:05:56

passed sexpots-freebsd #30056
FreeBSD

00:00:13

passed sexpots-linux #30045
Linux

00:00:23

passed sexpots-windows #30048
Windows

00:00:40

passed syncdraw-freebsd #30052
FreeBSD

00:00:30

passed syncdraw-linux #30053
Linux

00:01:06

passed syncterm-freebsd #30050
FreeBSD

00:01:07

passed syncterm-linux #30046
Linux

00:02:45

passed syncterm-windows #30051
FreeBSD

00:01:51