server: minor code cleanup

server: minor code cleanup

Suggested by Yuya after 77f9e95fe3c4, this is mostly using named values for
stdio descriptors.  The lockfile is also opened in binary mode when reading back
content from the child.

--- a/mercurial/server.py	Fri Mar 30 23:21:46 2018 -0400
+++ b/mercurial/server.py	Fri Mar 30 23:32:06 2018 -0400
@@ -55,8 +55,8 @@
             fd = os.open(postexecargs['unlink'],
                          os.O_WRONLY | os.O_APPEND | os.O_BINARY)
             try:
-                os.dup2(fd, 1)
-                os.dup2(fd, 2)
+                os.dup2(fd, procutil.stdout.fileno())
+                os.dup2(fd, procutil.stderr.fileno())
             finally:
                 os.close(fd)
 
@@ -94,7 +94,7 @@
                 # If the daemonized process managed to write out an error msg,
                 # report it.
                 if pycompat.iswindows and os.path.exists(lockpath):
-                    with open(lockpath) as log:
+                    with open(lockpath, 'rb') as log:
                         for line in log:
                             procutil.stderr.write(line)
                 raise error.Abort(_('child process failed to start'))
@@ -129,12 +129,14 @@
         if logfile:
             logfilefd = os.open(logfile, os.O_RDWR | os.O_CREAT | os.O_APPEND,
                                 0o666)
-        os.dup2(nullfd, 0)
-        os.dup2(logfilefd, 1)
-        os.dup2(logfilefd, 2)
-        if nullfd not in (0, 1, 2):
+        os.dup2(nullfd, procutil.stdin.fileno())
+        os.dup2(logfilefd, procutil.stdout.fileno())
+        os.dup2(logfilefd, procutil.stderr.fileno())
+        stdio = (procutil.stdin.fileno(), procutil.stdout.fileno(),
+                 procutil.stderr.fileno())
+        if nullfd not in stdio:
             os.close(nullfd)
-        if logfile and logfilefd not in (0, 1, 2):
+        if logfile and logfilefd not in stdio:
             os.close(logfilefd)
 
         # Only unlink after redirecting stdout/stderr, so Windows doesn't