preproc: Allow anonymous contexts
preproc: Allow anonymous contexts

Allow %push and %repl without a context name.  For a lot of uses, it
is only a potential source of namespace pollution.

techkey/nasm,techkey/nasm,letolabs/nasm,turingstudio/nasm,projedi/nasm,Distrotech/nasm,techkey/nasm,turingstudio/nasm,techkey/nasm,projedi/nasm,turingstudio/nasm,turingstudio/nasm,Distrotech/nasm,Distrotech/nasm,projedi/nasm,Distrotech/nasm,projedi/nasm,letolabs/nasm,letolabs/nasm,projedi/nasm,techkey/nasm