[FR] OpenBSD support

[ITwrx]

right now if i try to run ps_mem.py i get:

./ps_mem.py
Traceback (most recent call last):
  File "./ps_mem.py", line 120, in open
    return open(self.path(*args), errors='ignore')
FileNotFoundError: [Errno 2] No such file or directory: '/proc/sys/kernel/osrelease'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "./ps_mem.py", line 629, in <module>
    if __name__ == '__main__': main()
  File "./ps_mem.py", line 582, in main
    verify_environment(pids_to_show)
  File "./ps_mem.py", line 567, in verify_environment
    kernel_ver()
  File "./ps_mem.py", line 221, in kernel_ver
    kv = proc.open('sys/kernel/osrelease').readline().split(".")[:3]
  File "./ps_mem.py", line 126, in open
    raise LookupError
LookupError

Thanks