Fix fd leak and stale socket issues (security audit) - #11

1. Close socket on remote_init failure (com.c)
   When early connect succeeds but DBGp handshake fails, the socket
   was never closed — causing fd exhaustion under repeated failures.

2. Reset socket fd to -1 after close (com.c)
   xdebug_mark_debug_connection_not_active() closed the socket but
   left the old fd value, risking use-after-close if the fd is reused.

3. Add PHP_DEBUGGER_SESSION_START to RINIT pre-check (xdebug.c)
   The early connect pre-check only tested XDEBUG_SESSION_START env
   var but not the new alias, causing early connect to be skipped
   when using the new trigger name.