CursorWindow.cpp: replace dup() with fcntl(F_DUPFD_CLOEXEC)

Replace calls to dup() with fcntl(F_DUPFD_CLOEXEC). The only difference
between the two is that O_CLOEXEC is set on the newly duped file
descriptor. This helps address file descriptor leaks crossing an exec()
boundary.

Test: compiles and boots
Bug: 120983106
Change-Id: Icc2ff2f7f398905aa8283c8797898114d34a9829