memfd_create: move implementation to libc/gen

[brooks]

Summary

Due to memfd_create(3)'s construction of a path to pass to shm_open2(2),
it has a much larger than typical dependency footprint for a system
call wrapper (the list currently includes calloc, memset, sprintf, and
strlen). As such, split it off into it's own file under libc/gen
to lighten libc/sys's dependency list.

[imp]

Saw on github. Looks goud

[emaste]

lib/libc/gen/memfd_create.c

31–41

are any of these headers no longer needed in one or the other of these files?

[kevans]

No objection, lgtm

[brooks]

• tidy headers