Decode the O_PATH flag

* file.c (open_mode_flags): Add O_PATH.
Delete old PTRACE_{PEEK,POKE}USR logic

The code base has settled on PTRACE_{PEEK,POKE}USER (with an E) and has
logic in defs.h to make sure it's set sanely.  Delete this old logic as
the defs.h takes care of it now.

* process.c: Delete PTRACE_PEEKUSR/PTRACE_POKEUSR defines.
* signal.c: Likewise.
* syscall.c: Delete PTRACE_PEEKUSR define.
* util.c: Likewise.