security/sssd2: Assorted port cleanups

• Simplify depending on Kerberos
• - Do not use gssapi:bootstrap
• - Do not redefine variables already defined by USES
• - Instead of patching, pass the KRB5_CONFIG env var
• Trim unused dependencies
• Remove hunks from the configure.ac patch that aren't needed anymore
• Simplify SHEBANG_FILES
• No need to define LIB_DIRS, DEBUG_FLAGS and STRIP
• Bump PORTREVISION to catch possible regressions

Tested by: arrowd
Approved by: 0mp, jhixson
Pull Request: https://github.com/freebsd/freebsd-ports/pull/272
Sponsored by: Future Crew, LLC