math/frobby: Improve port and adopt again

• Reorder variables.
• Simplify port by introducing USES= localbase:ldflags.
• Set ALL_TARGET= # Empty to avoid defining do-build.
• Remove BINARY_ALIAS: patch Makefile instead.
• Always install executable, remove corresponding option, rename EXEDOCS option to DOCS.
• Always install shared library, remove corresponding option.
• Install static library with INSTALL_DATA and shared library with INSTALL_LIB.
• Improve shared library management by using an upstream patch. This also solves a poudriere warning about libfrobby.so.0 not having a SONAME.
• Remove outdated warnings about failing tests.
• Fix LIBDOCS dependencies.
• Remove pkg-help file, which does not contain any useful information any more.
• Fix pkg-plist.

Reviewed by: diizzy thierry
Approved by: gerald (mentor)
Differential Revision: https://reviews.freebsd.org/D35028