Adapt to findings from preseed.debian.net

 * Comment on questions not meeting the syntax.
 * Add period `.` as allowed character when matching fields, as those
   are actively used in question namnes.

After fetching all files from preseed.debian.net, the following:

```
cat **/* | sed -n 's/#\s*d-i\s*\([^ ]\+\).*/\1/p' |
grep -v '^\([[:alnum:]._-]\+/\)\+[[:alnum:]._-]\+$'
```

returns only the documented anomaly.

1 files changed, 27 insertions, 10 deletions

diff --git a/syntax/preseed.vim b/syntax/preseed.vim
index d716866..4690280 100644
--- a/syntax/preseed.vim
+++ b/syntax/preseed.vim
@@ -1,7 +1,7 @@
 " Vim syntax file
 " Language:	Debian preseed
 " Maintainer:	cos <cos>, https://www.netizen.se/#contact
-" Last Change:	2023 Oct 15
+" Last Change:	2023 Nov 2
 " Remark:	https://wiki.debian.org/DebianInstaller/Preseed
 
 " quit when a syntax file was already loaded
@@ -9,11 +9,28 @@ if exists('b:current_syntax')
   finish
 endif
 
-
 " The syntax of each line in a preseed file follows:
 " <owner> <question name> <question type> <value>
-
-" Create clusters for two of these four parts of a preseed line.
+" Lets sloppily refer to them as columns.
+
+" Out in the wild, there appears to be two preseed questions which fail to
+" match the syntax rules. According to preseed.debian.net, these two are:
+"
+"   debian-installer/shell-plugin and sitesummary-client/nagios-server
+"
+" The first one takes the `terminal` question type (rather than `boolean`,
+" `string`, `password`, `select`, `multiselect` or `note`). Attempting to ask
+" about this on #debian-boot gave the classic (non-)warm welcoming from one of
+" the regulars there. That person claimed to have investigated the issue and
+" stated that it is correct as it is. Followed up with a polite but clear
+" instruction to shut-up.
+"
+" The second one is described as an internal hidden debconf question, and has
+" 'db_get' injected between the owner and the question name fields. That could
+" either be a clever hack or a bug. No investigation has been done to figure
+" out which.
+
+" Create clusters for two of these four columns of a preseed line.
 syntax cluster preseedSecond contains=preseedQuestion,preseedOwnerLC
 syntax cluster preseedThird contains=preseedBooleanType,preseedStrLikeType,preseedMultiSelType,preseedNote,preseedSelectType,preseedQuestionLC
 
@@ -21,20 +38,20 @@ syntax cluster preseedThird contains=preseedBooleanType,preseedStrLikeType,prese
 " Define the highlighting. Doesn't map perfectly to the group's intents, but
 " arguably close enough?
 
-" First, <owner>
+" First column, <owner>
 hi! def link preseedOwner        Label
 
-" Second, <question name>
+" Second column, <question name>
 hi! def link preseedQuestion     Identifier
 
-" Third, <question type>
+" Third column, <question type>
 hi! def link preseedBooleanType  Type
 hi! def link preseedMultiSelType Type
 hi! def link preseedNote         Type
 hi! def link preseedSelectType   Type
 hi! def link preseedStrLikeType  Type
 
-" Fourth, <value>
+" Fourth column, <value>
 hi! def link preseedBooleanVal   Special
 hi! def link preseedMultiSelVal  Special
 hi! def link preseedStrLikeVal   String
@@ -54,10 +71,10 @@ hi! def link preseedStrLikeLC    Operator
 " Add the syntax matching rules.
 
 " First, <owner>
-sy match preseedOwner              /^\s*[[:alnum:]_-]\+\(\s\|$\)/ nextgroup=@preseedSecond,preseedOwnerLC
+sy match preseedOwner              /^\s*[[:alnum:]._-]\+\(\s\|$\)/ nextgroup=@preseedSecond,preseedOwnerLC
 
 " Second, <question name>
-sy match preseedQuestion contained "\s*\([[:alnum:]_-]\+/\)\+[[:alnum:]_-]\+\(\s\|$\)" nextgroup=@preseedThird,preseedQuestionLC
+sy match preseedQuestion contained "\s*\([[:alnum:]._-]\+/\)\+[[:alnum:]._-]\+\(\s\|$\)" nextgroup=@preseedThird,preseedQuestionLC
 
 " Third, <question type>
 " (boolean, multiselect, note, password, select, string)