Thanks to references from Brad Smith, BSDs use getmntinfo as
a system call for mounted filesystems.

Most BSDs return statfs structures, (and we use OSX's statfs64),
but NetBSD uses a statvfs structure.

Simplify Linux getmntent_r to just use getmntent.

AIX uses getmntent.

An attempt at writing Solaris compatibility with
a small bit of HPUX compatibility was made based on man page
entries only. Fixes welcome.

statvfs structures now use f_bsize for consistency with statfs

Test case adjusted as PATH_MAX is OS defined (e.g. 1023 on AIX)

Fixes: 0ee5cf83

also fixes:

MDEV-27818: Disk plugin does not show zpool mounted devices

This is because zpool mounted point don't begin with /.

Due to the proliferation of multiple filesystem types since this
was written, we restrict the entries listed in the disks plugin
to excude:
* read only mount points (no point monitoring, and
  includes squash, snaps, sysfs, procfs, cgroups...)
* mount points that aren't directories (excludes /etc/hostname and
  similar mounts in containers). (getmntent (Linux/AIX) only)
* exclude systems where there is no capacity listed (excludes various
  virtual filesystem types).

Reviewer: Sergei Golubchik