sysusers/provides: parse and output static IDs

This adds support for parsing static UIDs and GIDs from sysusers.d
fragments, and automatically forwarding them to the generated
'Provides' entries.
It will allow inspecting users/groups with static IDs directly
from package metadata:
```
$ rpm --query --provides --package gdm-41.0-3.fc36.x86_64.rpm
[...]
group(gdm) = 42
user(gdm) = 42
```

sysusers.prov

@@ -1,5 +1,40 @@
 #!/bin/bash
 
+process_u() {
+    if [ ! -z "${2##*[!0-9]*}" ]; then
+        # Single shared static ID.
+        echo "user($1) = $2"
+        echo "group($1) = $2"
+    elif [[ $2 == *:* ]]; then
+        # UID:<group>.
+        uid=$(echo $2 | cut -d':' -f1 -)
+        group=$(echo $2 | cut -d':' -f2 -)
+        if [ ! -z "${group##*[!0-9]*}" ]; then
+            # UID:GID.
+            echo "user($1) = ${uid}"
+            echo "group($1) = ${group}"
+        else
+            # UID:<groupname>.
+            echo "user($1) = ${uid}"
+            echo "group(${group})"
+        fi
+    else
+        # Dynamic (or something else uninteresting).
+        echo "user($1)"
+        echo "group($1)"
+    fi
+}
+
+process_g() {
+    if [ ! -z "${2##*[!0-9]*}" ]; then
+        # Static GID.
+        echo "group($1) = $2"
+    else
+        # Dynamic (or something else uninteresting).
+        echo "group($1)"
+    fi
+}
+
 parse() {
     while read line; do
         [ "${line:0:1}" = '#' -o "${line:0:1}" = ';' ] && continue
@@ -8,12 +43,10 @@ parse() {
         set -- $line
         case "$1" in
             ('u')
-                echo "user($2)"
-                echo "group($2)"
-                # TODO: user:group support
+                process_u "$2" "$3"
                 ;;
             ('g')
-                echo "group($2)"
+                process_g "$2" "$3"
                 ;;
             ('m')
                 echo "user($2)"