security/letskencrypt: Add letskencrypt port

[brnrd]

Summary

Proposed commit log message:

security/letskencrypt: Add letskencrypt port

letskencrypt is a client for Let's Encrypt users designed for
security written in C. This is the -portable version.

  - Use Github not kritaps.bsd.lv for versioned tarball

Reviewed_by:	koobs, sascha_root-login.org
Approved by:	
Differential_Revision:	D6649

Test Plan

• poudriere testport
• portlint -AC clean

[sascha_root-login.org]

I think i've been added by accident so i'll accept the revision to not stand in the way of the process :)

[brnrd]

In D6649#140299, @sascha_root-login.org wrote:

I think i've been added by accident so i'll accept the revision to not stand in the way of the process :)

You were added as one of the maintainers of letsencrypt related ports 😄

[brnrd]

Add USE_OPENSSL

• Fix spurious entry in security/Makefile

[brnrd]

Change directory and file locations

• All files/dirs in /usr/local as per hier(8)
• Change default paths in main.c
• Change default paths in letskencrypt(1)
• Add empty directories with secure modes to plist