sysutils/fusefs-mergerfs

filesystems: add new category for file systems and related utilities

The filesystems category houses file systems and file system utilities.
It is added mainly to turn the sysutils/fusefs-* pseudo-category into
a proper one, but is also useful for the sundry of other file systems
related ports found in the tree.

Ports that seem like they belong there are moved to the new category.
Two ports, sysutils/fusefs-funionfs and sysutils/fusefs-fusepak are
not moved as they currently don't fetch and don't have TIMESTAMP set
in their distinfo, but that is required to be able to push a rename
of the port by the pre-receive hook.

Approved by:	portmgr (rene)
Reviewed by:	mat
Pull Request:	https://github.com/freebsd/freebsd-ports/pull/302
PR:		281988

sysutils/fusefs-mergerfs: Add new port

mergerfs is a union filesystem geared towards simplifying storage and
management of files across numerous commodity storage devices. It is
similar to mhddfs, unionfs, and aufs.