* main.h: Create S2K_DIGEST_ALGO macro so we do not need to always set
opt.s2k_digest_algo.  This helps fix a problem with PGP 2.x encrypted
symmetric messages.  Change all callers (encode.c, g10.c, keyedit.c,
keygen.c, passphrase.c, sign.c).
* armor.c, cardglue.c, getkey.c, import.c, keygen.c: Be consistent in some
more quoted strings.  Always use 'user ID', not 'user id', "quotes" for
user IDs, etc.