refactor: build the librrd argument list without the dummy slot

[somethingwithproof]

rrd_args_init_by_phparray() is the shared path for all twelve functions, and two of the bugs in the branches below it lived here. The structure is the reason.

Slot 0 held a literal nothing ever read:

result->args[0] = "dummy";          /* string literal into a char * */
result->args[1] = estrdup(command_name);
...
rrd_fetch(argv->count - 1, &argv->args[1], ...);   /* every call site compensates */

void rrd_args_free(rrd_args *args) {
    for (i = 1; i < args->count; i++)   /* ownership encoded as a bare 1 */
        efree(args->args[i]);
}

So ownership was positional and unwritten: slot 0 borrowed, the rest owned, and nothing enforced it. This drops the slot, gives every element the same lifetime, and lets callers pass the array as it stands.

Also in here, since they were all consequences of the same shape:

• zend_hash_internal_pointer_reset / move_forward against a pre-captured count becomes ZEND_HASH_FOREACH_VAL, bounded by what was actually allocated.
• Each option was built into a smart_string, estrdup'd out of it, then the smart_string freed. One estrndup from the zend_string does it.
• Eleven call sites reported "cannot allocate arguments options" for what is nearly always a validation failure. The builder now says what went wrong, and php_error_docref names the function: RRDCreator::save(): options array must not be empty.
• The same char *argv[3] / "dummy" fossil in rrd_info() and rrd_lastupdate() is gone.

No behaviour change beyond the diagnostic text, which two tests are updated for.

Validation

33/33 on PHP 8.1, 8.4, 8.5 and 8.6.0-dev, zero compiler warnings on librrd 1.7.2 and 1.11.0, and clean under valgrind --leak-check=full.

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Stack order. This is part of a series, each building on the one before. Merge order:
#11 (test suite) → #12 (memory safety) → #13 (open_basedir, value semantics) → #14 (php-src master) → #15 (librrd argv) → #16 (argument builder) → #17 (autoconf). Commits below this PR's own belong to the ones underneath it.