bin/df: allow -t option to be used together with -l

The df command provides a -l option to exclude all non-local file
systems and a -t option with a (positive or negative) list of file
system types to display.

This commit adds support for a combination of -l and -t. If both are
specified, the parameter list of the -t option is applied on top of
the selection of öocal file systems (independently of the order of
the -l and -t options).

E.g., "df -t noprocfs,sysfs -l" will select all local file systems
except those of type procfs and sysfs.

PR: 260921
Approved by: imp
Relnotes: yes
MFC after: 1 month
Differential Revision: https://reviews.freebsd.org/D33748