libwebsockets

context: add info members to drop privileges using user and group name strings

Up until now if you wanted to drop privs, a numeric uid and gid had to be
given in info to control post-init permissions... this adds info.username
and info.groupname where you can do the same using user and group names.

The internal plat helper lws_plat_drop_app_privileges() is updated to directly use
context instead of info both ways it can be called, and to be able to return fatal
errors.

All failures to lookup non-0 or -1 uid or gid names from uid, or to look up
uid or gid from username or groupnames given, get an err message and fatal exit.