Getting partitions with hidepid=2 on proc fails

[rikvdh]

Describe the bug
When getting disk.Partitions on a system for an unprivileged user, whilst having hidepid=2 option on the proc-mount fails.

Behavior was broken by this commit: b39f8bb#diff-1c057146d22f660d0d6142cbda77095e8621efa99d8b4058bbf885c81e6e397e

/proc/1 might not be available. Perhaps a fallback to self can be added?

Environment (please complete the following information):

• Windows: [paste the result of ver]
• Linux: [paste contents of /etc/os-release and the result of uname -a]
• Mac OS: [paste the result of sw_vers and uname -a
• FreeBSD: [paste the result of freebsd-version -k -r -u and uname -a]
• OpenBSD: [paste the result of uname -a]

[Lomanic]

Adding a fallback to /proc/self is tracked in #1159

A workaround is mentioned in #1024 (comment) (adding user to a specific group and mount procfs with gid option)