Fix copyfile() not preserving source mode with restrictive umask

copyfile() passes the source mode to open(2), which applies the umask,
so a 0644 file copied with umask 0027 ends up 0640.

Apply the source mode with fchmod() after creating the destination.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

ChangeLog.md

@@ -8,6 +8,8 @@ All notable changes to the project are documented in this file.
 --------------
 
 ### Fixes
+- Fix `copyfile()` not preserving the source file mode when the process
+  umask is more restrictive
 - Fix `mkpath()`, and `fmkpath()`, not applying the given mode when the
   process umask is more restrictive, e.g., requesting 0755 with umask
   0027 silently created a 0750 directory