directory(3): Add an ERRORS section

• Add an ERRORS section for opendir(3) and closedir(3)
• Document also the errors of readdir(3), readdir_r(3) and telldir(3)
• Convert the code sample into an EXAMPLES section

PR: 75711

Submitted by: abc <abc at ai1 dot anchorage dot mtaonline dot net>
Reviewed by: 0mp, bcr, jilles
Approved by: 0mp, bcr, jilles
Obtained from: partial from OpenBSD
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D25892