parse-util: Add parse_capability_set()

Let's extract common capability parsing code into a generic function
parse_capability_set() with a comprehensive set of unit tests.

We also replace usages of UINT64_MAX with CAP_MASK_UNSET where
applicable and replace the default value of CapabilityBoundingSet
with CAP_MASK_ALL which more clearly identifies that it is initialized
to all capabilities.

AI (copilot) was used to extract the generic function and write the
unit tests, with manual review and fixing afterwards to make sure
everything was correct.

src/basic/parse-util.c

@@ -7,6 +7,8 @@
 #include <sys/socket.h>
 
 #include "alloc-util.h"
+#include "capability-list.h"
+#include "capability-util.h"
 #include "errno-list.h"
 #include "extract-word.h"
 #include "locale-util.h"
@@ -809,3 +811,39 @@ bool nft_identifier_valid(const char *id) {
 
         return in_charset(id + 1, ALPHANUMERICAL "/\\_.");
 }
+
+int parse_capability_set(const char *s, uint64_t initial, uint64_t *current) {
+        int r;
+
+        assert(s);
+        assert(current);
+
+        if (isempty(s)) {
+                *current = CAP_MASK_UNSET;
+                return 1;
+        }
+
+        bool invert = false;
+        if (s[0] == '~') {
+                invert = true;
+                s++;
+        }
+
+        uint64_t parsed;
+        r = capability_set_from_string(s, &parsed);
+        if (r < 0)
+                return r;
+
+        if (parsed == 0 || *current == initial)
+                /* "~" or uninitialized data -> replace */
+                *current = invert ? all_capabilities() & ~parsed : parsed;
+        else {
+                /* previous data -> merge */
+                if (invert)
+                        *current &= ~parsed;
+                else
+                        *current |= parsed;
+        }
+
+        return r;
+}