unify the use of +threads as a variant name

[mojca (Mojca Miklavec)]

From #43593: would it make sense to unify the variant names used across different ports?

Here are some variant names from different ports:

boost:     variant no_single  description {Disable building single-threaded libraries}
gauche:    variant no_threads { configure.args-delete --enable-threads=pthreads }
raxml:     variant pthreads   conflicts hybrid description {Pthreads implementation}
cherokee:  variant no_pthread description {Disable threading support}

sbcl:      variant threads    description {Enable multi-threaded runtime using the Mach pthreads interface.}
tcl:       variant threads    description {add multithreading support}
yap:       variant threads
abinit:    variant threads    description {Build with support for multi-thread support (openMP)}
openmpi:   variant threads    description {enable threads for MPI applications}
wannier90: variant threads    description {Build with threaded ATLAS}

hdf5:      variant threadsafe description {Enable threadsafety (experimental, fails unit-tests)}

If this doesn't apply to your port (that is: if the keyword +threads misses the point – I'm not sure if +threads and +threadsafe have "compatible" meanings for example), please say so and remove your port from the list of affected ports. But variants like +no_threads should certainly be inverted.