Rollup merge of #137494 - nabijaczleweli:dup, r=Mark-Simulacrum

libstd: init(): dup() subsequent /dev/nulls instead of opening them again

This will be faster, and also it deduplicates the code so win/win

The dup() is actually infallible here. But whatever.

Before:
```
poll([{fd=0, events=0}, {fd=1, events=0}, {fd=2, events=0}], 3, 0) = 1 ([{fd=2, revents=POLLNVAL}])
openat(AT_FDCWD, "/dev/null", O_RDWR)   = 2
rt_sigaction(SIGPIPE, {sa_handler=SIG_IGN, sa_mask=[PIPE], sa_flags=SA_RESTORER|SA_RESTART, sa_restorer=0x7f5749313050}, {sa_handler=SIG_DFL, sa_mask=[], sa_flags=0}, 8) = 0

poll([{fd=0, events=0}, {fd=1, events=0}, {fd=2, events=0}], 3, 0) = 2 ([{fd=0, revents=POLLNVAL}, {fd=2, revents=POLLNVAL}])
openat(AT_FDCWD, "/dev/null", O_RDWR)   = 0
openat(AT_FDCWD, "/dev/null", O_RDWR)   = 2
rt_sigaction(SIGPIPE, {sa_handler=SIG_IGN, sa_mask=[PIPE], sa_flags=SA_RESTORER|SA_RESTART, sa_restorer=0x7efe12006050}, {sa_handler=SIG_DFL, sa_mask=[], sa_flags=0}, 8) = 0

poll([{fd=0, events=0}, {fd=1, events=0}, {fd=2, events=0}], 3, 0) = 3 ([{fd=0, revents=POLLNVAL}, {fd=1, revents=POLLNVAL}, {fd=2, revents=POLLNVAL}])
openat(AT_FDCWD, "/dev/null", O_RDWR)   = 0
openat(AT_FDCWD, "/dev/null", O_RDWR)   = 1
openat(AT_FDCWD, "/dev/null", O_RDWR)   = 2
rt_sigaction(SIGPIPE, {sa_handler=SIG_IGN, sa_mask=[PIPE], sa_flags=SA_RESTORER|SA_RESTART, sa_restorer=0x7fc2dc7ca050}, {sa_handler=SIG_DFL, sa_mask=[], sa_flags=0}, 8) = 0
```

After:
```
poll([{fd=0, events=0}, {fd=1, events=0}, {fd=2, events=0}], 3, 0) = 1 ([{fd=1, revents=POLLNVAL}])
openat(AT_FDCWD, "/dev/null", O_RDWR)   = 1
rt_sigaction(SIGPIPE, {sa_handler=SIG_IGN, sa_mask=[PIPE], sa_flags=SA_RESTORER|SA_RESTART, sa_restorer=0x7f488a3fb050}, {sa_handler=SIG_DFL, sa_mask=[], sa_flags=0}, 8) = 0

poll([{fd=0, events=0}, {fd=1, events=0}, {fd=2, events=0}], 3, 0) = 2 ([{fd=1, revents=POLLNVAL}, {fd=2, revents=POLLNVAL}])
openat(AT_FDCWD, "/dev/null", O_RDWR)   = 1
dup(1)                                  = 2
rt_sigaction(SIGPIPE, {sa_handler=SIG_IGN, sa_mask=[PIPE], sa_flags=SA_RESTORER|SA_RESTART, sa_restorer=0x7f1a8943c050}, {sa_handler=SIG_DFL, sa_mask=[], sa_flags=0}, 8) = 0

poll([{fd=0, events=0}, {fd=1, events=0}, {fd=2, events=0}], 3, 0) = 3 ([{fd=0, revents=POLLNVAL}, {fd=1, revents=POLLNVAL}, {fd=2, revents=POLLNVAL}])
openat(AT_FDCWD, "/dev/null", O_RDWR)   = 0
dup(0)                                  = 1
dup(0)                                  = 2
rt_sigaction(SIGPIPE, {sa_handler=SIG_IGN, sa_mask=[PIPE], sa_flags=SA_RESTORER|SA_RESTART, sa_restorer=0x7f4e3a4c7050}, {sa_handler=SIG_DFL, sa_mask=[], sa_flags=0}, 8) = 0
```

Author: jhpratt