* configure.ac (COMMON_CFLAGS): New variable.  AC_SUBST it.
(COMMON_LIBS): Likewise.  AC_SUBST it.
(LIBSECRET_CFLAGS): Likewise.
(LIBSECRET_LIBS): Likewise.
(--enable-libsecret): Add option to enable support for libsecret.  If
enabled, check for its presense.
* pinentry/password-cache.h: New field.
* pinentry/password-cache.c: New field.
* pinentry/pinentry.h (struct pinentry): Add fields pin_from_cache,
allow_external_password_cache, tried_password_cache, keyinfo, and
may_cache_password.
* pinentry/pinentry.c: Include "password-cache.h".
(pinentry): Initialize new fields.
(option_handler): Handle the "allow-external-password-cache" option.
(cmd_setkeyinfo): Implement it.
(cmd_getpin): Read the password from the cache, if appropriate.  Save
it to the cache, if appropriate.
* pinentry/Makefile.am (AM_CPPFLAGS): Add $(COMMON_CFLAGS).
(LDADD): Add $(COMMON_LIBS).
(libpinentry_a_SOURCES): Add password-cache.h
password-cache.c.
* gtk+-2/pinentry-gtk-2.c (may_save_passphrase_toggled): New function.
(create_window): Take additional parameter, the pinentry's context.
Update callers.
[HAVE_LIBSECRET]: Show a checkbox asking whether the passphrase should
be saved.
* gtk+-2/Makefile.am (AM_CPPFLAGS): Add $(COMMON_CFLAGS).
(LDADD): Add $(COMMON_LIBS).
* curses/Makefile.am (AM_CPPFLAGS): Add $(COMMON_CFLAGS).
(LDADD): Add $(COMMON_LIBS).
* tty/Makefile.am (AM_CPPFLAGS): Add $(COMMON_CFLAGS).
(LDADD): Add $(COMMON_LIBS).
* doc/pinentry.texi (Protocol): Update documentation.  Describe the
protocol and provide some justification.