linux: os2._process_start: skip non-ELF files when searching in PATH

[jkenda]

os2.process_start was trying to execute bash scripts that were flagged executable so I added a check for the ELF magic.

[jasonKercher]

I believe you want it to execute bash scripts in $PATH? It currently fails to because of the CLOEXEC flag on the descriptor causes ENOENT. So, for bash scripts we want to leak the file descriptor, and it currently does not.

I snuck a solution for this into #4600 and made it name based (maybe I should have made it separate). I had trouble getting execveat with that AT_EMPTY_PATH flag to work within a container. So, maybe the issue is more complex than just elf vs shebang? The name based approach just works in all cases and makes the file descriptor the kernel's problem.