i#2884: Add O_CLOEXEC to prevent fd leakage in injector

[mmustafasenoglu]

i#2884: Add O_CLOEXEC to prevent fd leakage in injector

When drrun injects libdynamorio.so into a child process, the file
descriptor used to open the library was not marked close-on-exec.
This caused the fd to leak into the target program, which could be
exploited by malware to detect DynamoRIO or alter behavior.

Added O_CLOEXEC flag to the injectee_open call in
core/unix/injector.c.

Reproduction:

1. Run: drrun -- target_program
2. Check /proc/<target_pid>/fd before fix - fd 3 points to
   libdynamorio.so
3. After fix, fd 3 is closed on exec and not visible in
   the target process

Testing:
Verified that the fd is closed on exec by checking
/proc//fd after running drrun. The fd no longer leaks
to the target process.

Fixes #2884

[edeiana]

Thank you for contributing!

Please clean up your commit message (i.e., PR description) and title following: https://dynamorio.org/page_code_reviews.html#sec_commit_messages.

For example: stay within 80 chars per line; the tile should be prefixed with i#2884; no need to add a diff of the changes in the PR description (that's what git diff is for); end your PR description with Fixes #2884; avoid markdown titles within the PR description (this change is small enough to not need subsections like: "Description", "Change", "Testing").

If adding a test for this change is not feasible, add the steps used to reproduce the issue showing that it's actually fixed in the PR description; the description under "Testing" is not enough, please add the precise commands and output.