Merge bitcoin/bitcoin#26809: compat: use STDIN_FILENO over 0

585c672212 compat: use STDIN_FILENO over 0 (fanquake)

Pull request description:

  This is already used throughout this file, and is self-documenting.

ACKs for top commit:
  john-moffett:
    ACK 585c672212
  achow101:
    ACK 585c672212
  hebasto:
    ACK 585c672212, I have reviewed the code and it looks OK, I agree it can be merged.
  kristapsk:
    utACK 585c672212
  aureleoules:
    ACK 585c672212

Tree-SHA512: c0114ae896ba5404be70b804ee9f454d213f1d789c8f5a578c422dd15a308a214e6851fee76c0ec736a212bc86fb33ec17af1b22e5d23422c375ca4458251356

src/compat/stdin.cpp

@@ -58,7 +58,7 @@ bool StdinReady()
     return false;
 #else
     struct pollfd fds;
-    fds.fd = 0; /* this is STDIN */
+    fds.fd = STDIN_FILENO;
     fds.events = POLLIN;
     return poll(&fds, 1, 0) == 1;
 #endif