nfs: Add support for the new _PC_CASE_INSENSITIVE name

Commit d6864221d8 added support for the _PC_CASE_INSENSITIVE
name for pathconf, to indicate if case insensitive lookups
are configured.

This patch adds support for it for both NFSv4 and
NFSv3.

It also adds a fsidp argument to nfsv4_fillattr() which
will be used in future commit(s) related to NFSv4.n
directory delegations.

(cherry picked from commit e5aa60d0695803c83dc0af08c057b82c463da3f2)