pathconf: Define a new name called _PC_CASE_INSENSITIVE

Some FreeBSD file systems (msdosfs and optionally ZFS)
handle case insensitive file name lookup.
NFS (v3 and 4) provide an attribute to indicate this
capability, but without this patch, the NFS server does
not know if the file system does support this.

This patch adds the pathconf name_PC_CASE_INSENSITIVE
that can be used by the NFS server or user programs to
check for this feature.

(cherry picked from commit d6864221d88425a079f76ca106f30812c671ca9a)