MFH: r439169, fix fdescfs being enabled by default

Make bash use of fdescfs use optional, disabled by default, and forced off when
disabled.

This allows users to avoid mounting an extra FS that we don't enable by default.

Only display pkg-message with fdescfs instructions when option is enabled.

Submitted by: swills
Differential Revision: https://reviews.freebsd.org/D10394

Approved by: ports-secteam (with hat)