dlopen(3): mention fdlopen for capsicum(4)

Capsicum-sandboxed applications generally cannot use dlopen, as absolute
and cwd-relative paths cannot be accessed. Mention that fdlopen is
useful for sandboxed applications.

PR: 277169
Reviewed by: markj, oshogbo
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D45108