net/c3270: Make it a slave port of x11/x3270

[kbowling]

Summary

This merges net/c3270 into a slave port of x11/x3270 to ease maintenance since they are effectively the same source with different build options. c3270 will install the non-X11 components.

Test Plan

Passes poudriere testport

[timur]

Looks good to me, but as I don't have dev environment working right now, can't test.

Hope, you've tested it and it passes portlint :)