Reset SCgiTask m_trusted on connection reuse

SCgiTask objects are pre-allocated in a pool (scgi.cc) and reused across
SCGI connections. SCgiTask::open() did not reset m_trusted, so when a
task that had handled an untrusted connection (m_trusted=false) was
reused for a new connection, m_trusted stayed false unless the new
connection explicitly sent UNTRUSTED_CONNECTION=1.

The header parser only set m_trusted=false on value 1 and was a no-op
on value 0 (the comment said "default is trusted, so do nothing") —
which is wrong for a reused task that is no longer in default state.

This caused intermittent rejection of trusted commands (e.g. ruTorrent
calling execute.capture for UID detection) with "Command X is not allowed
for untrusted connections", producing cascading plugin failures and
"ruTorrent cannot determine the UID of rTorrent user" in the web UI.

Fix:
- SCgiTask::open() resets m_trusted=true to default.
- parse_headers explicitly sets m_trusted=true on UNTRUSTED_CONNECTION=0,
  so the value sent on the wire is authoritative regardless of pool
  reuse semantics.

Verified on gb4 with rtorrent 0.16.11 + this fix: 30/30 trusted calls
succeed, 30/30 untrusted correctly blocked, 30/30 trusted-after-untrusted
batch all succeed (previously 70%+ would fail in the same scenario).

src/rpc/scgi_task.cc

@@ -35,6 +35,10 @@ SCgiTask::open(SCgi* parent, int fd) {
   m_content_length      = 0;
   m_content_type        = XML;
   m_accepts_compression = false;
+  m_trusted             = true;  // SCgiTask is pooled and reused; reset trust to default
+                                 // so a prior untrusted connection does not leak its
+                                 // m_trusted=false into the next reuse, given that the
+                                 // UNTRUSTED_CONNECTION=0 parse branch is a no-op.
 
   torrent::this_thread::poll()->open(this);
   torrent::this_thread::poll()->insert_read(this);
@@ -231,7 +235,8 @@ SCgiTask::parse_headers(const char* current, unsigned int header_length) {
       if (std::strncmp(value, "1", 1+1) == 0)
         m_trusted = false;
       else if (std::strncmp(value, "0", 1+1) == 0)
-        ; // Default is trusted, so do nothing.
+        m_trusted = true;  // Explicit reset (open() also resets, but be defensive in case
+                           // future refactors stop pooling tasks or skip the open() reset).
       else
         return false;
     }