scd: Support PC/SC for "getinfo reader_list".

* scd/apdu.c: Include membuf.h.
(pcsc): Add reader_list field.
(open_pcsc_reader): Fill that field.
(apdu_get_reader_list): New.
* scd/command.c: Remove header ccid-driver.h.
(pretty_assuan_send_data): New.
(cmd_getinfo): Print all reader names.

Note that depending on the card backend (ccid or PC/SC) it might be
necessary to first send a reset followed by SERIALNO to get an updated
list of reader. Or well send KILLSCD.

The pretty printing of Assuan data lines does only work if you connect
direct to scdaemon because the wrapper in gpg-agent does not know
about this and combines the Assuan lines again.

• Signed-off-by: Werner Koch <wk@gnupg.org>