Rename the FT_BYTES separator types to SEP_ from BASE_.

Rename the FT_BYTES separator types to SEP_ from BASE_.

There's no requirement for "display" field values to have names
beginning with BASE_; use SEP_, to indicate that they're separators.

(And, yes, we should introduce SEP_NONE, with a value of 0, and use it.)

While we're at it, fix BASE_SEMICOLON - the separator is a colon, not a
semicolon - and document SEP_SPACE in doc/README.dissector.

Change-Id: I856aafda2c60b8320dffe287f0deb06a9604a2bf
Reviewed-on: https://code.wireshark.org/review/6486
Reviewed-by: Guy Harris <guy@alum.mit.edu>