Filter Valgrind FDs from getdents syscalls

Filter Valgrind FDs from getdents syscalls

This change prevents client programs from seeing Valgrind's internal file
descriptors when scanning /proc/self/fd or /proc/<pid>/fd.

This patch modifies the getdents and getdents64 syscall wrappers to
selectively filter out Valgrind's internal file descriptors only when
listing /proc/*/fd directories for the current process.

Add none/tests/getdents_filter.vgtest test that tests that the
Valgrind's file descriptors are hidden from the client program
and verifies both /proc/self/fd filtering and that regular
directory listings remain unfiltered.

https://bugs.kde.org/show_bug.cgi?id=331311