Make File.Move(..., ..., overwrite: true) more efficient on Unix.

[gukoff]

The current implementation has an inefficiency. When moving a file across volumes on Unix, and when the target file doesn't exist, the "rename" syscall will be called twice (unsuccesfully) instead of once.

By branching on "overwrite" from the beginning, we avoid calling the first "rename" in vain.

As a bonus, we avoid 2 unnecessary lstat syscalls.

From strace output:

stat("/tmp/dockervol/somedata", {st_mode=S_IFREG|0644, st_size=0, ...}) = 0
lstat("/tmp/dockervol/somedata", {st_mode=S_IFREG|0644, st_size=0, ...}) = 0              <-------- redundant
lstat("/somedata", 0x7ffc9d5abcd0)      = -1 ENOENT (No such file or directory)           <-------- redundant
mprotect(0x7f01a667e000, 4096, PROT_READ|PROT_WRITE) = 0
madvise(0x7f01a667e000, 4096, MADV_DODUMP) = 0
rename("/tmp/dockervol/somedata", "/somedata") = -1 EXDEV (Invalid cross-device link)     <-------- double work
rename("/tmp/dockervol/somedata", "/somedata") = -1 EXDEV (Invalid cross-device link)     <--------ᐟ 
lstat("/usr", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0
lstat("/usr/share", {st_mode=S_IFDIR|0755, st_size=4096, ...}) = 0

[dnfadmin]

All CLA requirements met.

[stephentoub]

Seems reasonable. Thanks.