mount(8): drop redundant filesystem-specific sections

mount(8): drop redundant filesystem-specific sections

Add a new section to the top of the filesystem-specific chapter to point
people generally to the filesystem-specific man pages.  This way we can
delete all the redundant subsections that say the same thing.

These subsections are deleted because they offer no options.  Which is
the same as all the filesystems the kernel supports but this man page
doesn't explicitly list (of which there are a few).
  coherent minix ramfs romfs squashfs sysv xenix

These subsections are deleted because they simply point to the respective
<fs>(5) or mount.<fs>(8) page which we now document at the top.  Some also
discuss the syscall ABI, but that doesn't seem appropriate for this page.
  btrfs cifs ext2 ext3 ext4 nfs nfs4 proc smbfs tmpfs xfs

Signed-off-by: Mike Frysinger <vapier@gentoo.org>